Client-side: 
addEventHandler("onClientResourceStart",resourceRoot, 
    function() 
        GUIEditor_Window = {} 
        GUIEditor_Button = {} 
        GUIEditor_Image = {} 
  
        GUIEditor_Window[1] = guiCreateWindow(224,131,364,323,"Umbrella Corperation's Weapon Shop!",false) 
        guiSetVisible(GUIEditor_Window[1], false) 
        guiSetAlpha(GUIEditor_Window[1],1) 
        GUIEditor_Window[2] = guiCreateWindow(0.25,0.35,0.5,0.2,"UPS!",true) 
        button = guiCreateButton (0.35,0.6,0.3,0.2,"ok",true,GUIEditor_Window[2]) 
        guiSetAlpha(GUIEditor_Window[2],1)     
        label = guiCreateLabel (0.35,0.25,0.5,0.2,"Not enough money for that weapon",true,GUIEditor_Window[2]) 
        guiSetVisible(GUIEditor_Window[2], false) 
        GUIEditor_Button[1] = guiCreateButton(19,39,80,45,"Buy Dildo Large: $500000",false,GUIEditor_Window[1]) 
        GUIEditor_Button[2] = guiCreateButton(144,37,80,45,"Buy Dildo Small: $250000",false,GUIEditor_Window[1]) 
        GUIEditor_Button[3] = guiCreateButton(266,36,80,45,"Buy 50 RPG: $15000",false,GUIEditor_Window[1]) 
        GUIEditor_Button[4] = guiCreateButton(145,119,79,50,"Buy 1000 Pistol Skill: $500000",false,GUIEditor_Window[1]) 
        GUIEditor_Button[5] = guiCreateButton(266,120,85,51,"Buy 1000 Dual Uzi Skill: $1000000",false,GUIEditor_Window[1]) 
        GUIEditor_Button[6] = guiCreateButton(17,120,79,50,"Buy 1000 SawnOff Skill: $750000",false,GUIEditor_Window[1]) 
        GUIEditor_Image[1] = guiCreateStaticImage(14,196,88,73,"images/molotov.png",false,GUIEditor_Window[1]) 
        GUIEditor_Button[7] = guiCreateButton(29,271,59,43,"Buy 5 Cocktales: $60000",false,GUIEditor_Window[1]) 
        GUIEditor_Image[2] = guiCreateStaticImage(273,190,75,80,"images/minigun.png",false,GUIEditor_Window[1]) 
        GUIEditor_Button[8] = guiCreateButton(280,271,70,43,"Buy 10000 Miniguns:  $100000",false,GUIEditor_Window[1]) 
        GUIEditor_Button[9] = guiCreateButton(114,200,145,99,"EXIT",false,GUIEditor_Window[1]) 
    end 
) 
  
addEventHandler("onClientGUIClick",root,function()--You've triggered onClientGUIClick, the source of the event is the button that the player/client clicked 
        if (source == GUIEditor_Button[29]) then 
            guiSetVisible (GUIEditor_Window[1],false) 
            showCursor (false) 
        elseif (source == GUIEditor_Button[9]) then 
            guiSetVisible (GUIEditor_Window[1],false) 
            showCursor (false) 
        elseif (source == GUIEditor_Button[30]) then 
            guiSetVisible (GUIEditor_Window[2],false) 
            showCursor (false) 
        elseif (source == GUIEditor_Button[1]) then 
            money = getPlayerMoney (localPlayer) 
            if (money >= 500000) then 
            outputChatBox ("You bought a Dildo Large! For: $500000",225,225,0) 
            takePlayerMoney(500000) 
            triggerServerEvent ("give10",getLocalPlayer(),give10) 
            else 
            guiSetVisible(GUIEditor_Window[2],true) 
            guiSetVisible(GUIEditor_Window[1],false) 
            end 
    elseif (source == GUIEditor_Button[2]) then 
        money = getPlayerMoney (localPlayer) 
        if (money >= 250000) then 
            outputChatBox ("You bought a Dildo Small! For: $250000",225,225,0) 
            takePlayerMoney(250000) 
            triggerServerEvent ("give11",getLocalPlayer(),give11) 
        else 
            guiSetVisible(GUIEditor_Window[1],false) 
            guiSetVisible(GUIEditor_Window[2],true) 
        end 
    elseif (source == GUIEditor_Button[3]) then 
        money = getPlayerMoney (localPlayer) 
        if (money >= 150000) then 
            outputChatBox ("You bought a... Seriously?! Nice try Troll!! For: $150000",225,225,0) 
            takePlayerMoney(150000) 
            triggerServerEvent ("give15",getLocalPlayer(),give15) 
        else 
            guiSetVisible(GUIEditor_Window[1],false) 
            guiSetVisible(GUIEditor_Window[2],true) 
        end 
    elseif (source == GUIEditor_Button[8]) then 
        money = getPlayerMoney (localPlayer) 
        if (money >= 100000) then 
            outputChatBox ("You Nab You Shouldnt Be A Troll! For: $100000",225,225,0) 
            takePlayerMoney(100000) 
            triggerServerEvent ("give14",getLocalPlayer(),give15) 
        else 
            guiSetVisible(GUIEditor_Window[1],false) 
            guiSetVisible(GUIEditor_Window[2],true) 
        end 
    elseif (source == GUIEditor_Button[7]) then 
        money = getPlayerMoney (localPlayer) 
        if (money >= 60000) then 
            outputChatBox ("You bought 5 Cocktails! For: $60000",225,225,0) 
            takePlayerMoney(60000) 
            triggerServerEvent ("give18",getLocalPlayer(),give18) 
        else 
            guiSetVisible(GUIEditor_Window[1],false) 
            guiSetVisible(GUIEditor_Window[2],true) 
        end 
    elseif (source == GUIEditor_Button[5]) then 
        money = getPlayerMoney (localPlayer) 
        if (money >= 1000000) then 
            outputChatBox ("You bought Dual Uzi Skill For: $1000000",225,225,0) 
            takePlayerMoney(1000000) 
            triggerServerEvent ("setPedStat75",getLocalPlayer()) -- you don't have to put the function as it is on the server-side and the event is attached to the function server side 
        else 
            guiSetVisible(GUIEditor_Window[1],false) 
            guiSetVisible(GUIEditor_Window[2],true) 
        end 
    elseif (source == GUIEditor_Button[4]) then 
        money = getPlayerMoney (localPlayer) 
        if (money >= 500000) then 
            outputChatBox ("You bought Dual Pistol Skill For: $500000",225,225,0) 
            takePlayerMoney(500000) 
            triggerServerEvent ("setPedStat69",getLocalPlayer()) 
        else 
            guiSetVisible(GUIEditor_Window[1],false) 
            guiSetVisible(GUIEditor_Window[2],true) 
        end 
    elseif (source == GUIEditor_Button[6]) then 
        money = getPlayerMoney (localPlayer) 
        if (money >= 750000) then 
            outputChatBox ("You bought Dual Sawn Off Skill For: $750000",225,225,0) 
            takePlayerMoney(750000) 
            triggerServerEvent ("setPedStat73",getLocalPlayer()) 
        else 
            guiSetVisible(GUIEditor_Window[1],false) 
            guiSetVisible(GUIEditor_Window[2],true) 
        end 
    elseif (source == button) then 
        guiSetVisible(GUIEditor_Window[2],false) 
        guiSetVisible(GUIEditor_Window[1],true) 
    elseif (source == button) then 
        guiSetVisible(GUIEditor_Window[2],false) 
        guiSetVisible(GUIEditor_Window[1],true) 
    end 
end) 
  
addEvent ("showGUI",true) 
addEventHandler ("showGUI",root,function() 
    guiSetVisible(GUIEditor_Window[1],true) 
    showCursor(true) 
end) 
 
BTW, The source of the event "onClientGUIClick" is the button or the element the client/local player clicked: onClientGUIClick 
Server-side: 
marker1 = createMarker (295.239,-37.58,1000.515,"cylinder",1.5,0,0,225,100) 
marker2 = createMarker (295.48,-80.38,1000.51,"cylinder",1.5,0,0,225,100) 
marker3 = createMarker (290.28,-109.35,1000.51,"cylinder",1.5,0,0,225,100) 
marker4 = createMarker (211.25,1858.46,12.10,"arrow",1.5,0,0,225,100) 
  
pedammusf = createPed (179,290.187,-111.514,1001.51) 
setElementInterior(pedammusf,6) 
setPedFrozen(pedammusf,true) 
  
pedammulv = createPed (179,295.47,-82.53,1001.51)   
setElementInterior (pedammulv,4) 
  
setElementInterior (marker2,4) 
setElementInterior (marker1,1) 
setElementInterior (marker3,6) 
  
  
addEventHandler ("onMarkerHit",root,function(jugador) 
    if (source == marker1) then 
        triggerClientEvent (jugador,"showGUI",root) 
    elseif (source == marker2) then 
        triggerClientEvent (jugador,"showGUI",root) 
    elseif (source == marker3) then 
        triggerClientEvent (jugador,"showGUI",root) 
    elseif (source == marker4) then 
        triggerClientEvent (jugador,"showGUI",root) 
    end 
end) 
  
addEvent("give10",true) 
addEventHandler("give10",root,function() 
giveWeapon (client,10,1,true) 
end) 
  
addEvent("give11",true) 
addEventHandler("give11",root,function() 
giveWeapon (client,11,1,true) 
end) 
  
  
  
addEvent("give15",true) 
addEventHandler("give15",root,function() 
giveWeapon (client,15,1,true) 
end) 
  
  
  
addEvent("give14",true) 
addEventHandler("give14",root,function() 
setWeaponProperty (client,14,1,true) 
end) 
  
addEvent("setPedStat75",true) 
addEventHandler("seTPedStat75",root,function() 
setPedStat (client,75,999) 
end) 
  
addEvent("setPedStat73",true) 
addEventHandler("setPedStat73",root,function() 
setPedStat (client,73,999) 
end) 
  
addEvent("setPedStat69",true) 
addEventHandler("setPedStat69",root,function() 
setPedStat (source,69,999) 
end)