[Enforcer501]

You could use an array, but I personally prefer sticking to defines for dialog ID's or listitem's
I.E.,
#define Item_GPS 0 //Item ID 0
#define Item_ExtraThinCondoms 1 //Item ID 1
if(listitem == Item_GPS)
{
SendClientMessage(playerid,COLOR_RED,"Oh, you need a GPS? Too bad.");
}
etc. You get the idea.