Well, you shouldn't accept the trade unless you have the Tektek ready at hand.
If, however, the customer is offline and completes the trade when they next get on and you happen to be offline at that time, that is different.
If that situation arises, once you have accepted the trade, you should sate so in the thread you're going offline and will post the Tektek after you come back.
... does that make sense? @-@