I think its fine. The real issue is that GM's can't intervene in the case one or the other gets scammed during the transaction though. Even if the agreement is made and logged ingame via /tells were talking about two separate worlds, possibly 4 separate characters, which i imagine is something the GM's can't monitor.

However, there is also an official premise that things that GM's can't intervene or monitor tend to fall under unacceptable behavior. For example, server wide lottery games run by players.