Make a timer, Add some code that when 30 minutes (which is going to be like 3000+ maybe) then find your trade code and add that code to it.
ie.
int tradetimer = 0;
if (<tradecodehere> && tradetimer == 30000){
trade code
}
Dig?
Also might want to add that it saves it to there user account. cause if you just do itlike thiss every time they log in they wont be able to trade for 30 minutes.