You should have said "Ok i accept now" when you see the trade and he has a chance to switch stands when you accept and then you check him again and if he doesn't have the stand you want, dont trade with him, but if he has the stand you want, accept it.
They should also make a trading history thing so incase you get scammed, you can just show proof of the trade that you got scammed on and they will ban the scammer and give you your stand back.