A system that promotes spam? This will likely not turn out very well, it will just make trading more frustrating.
An Auction house however would be great, just like we know it from other games. (WoW, Diablo, etc).
This way you could outright buy the pokemon from the auction house, or if you would like to negotiate for a better price, simply whisper the person selling the item(name should be available next to auction), or send an ingame mail.