Don't realy think it's needed...
Trades are fine even when you have to wait sometimes. Esp. since set trades don't run away.
It's not the problem of the trades/lag, it's the impatience or... *searches word* "promisebreaking" (does that word even exist?) of the other party that is the problem there. Xd
(hint, always have a junk already with you ;3)
But -if- they get invented i agree with Reeses, an accept/reject feature would be needed. Also for the sender of the Creatu some "cancel" button... esp for cases if you typed/copied the username wrong for some reason.