How it works:
Seller A sends Buyer B (who's currently offline) a trade offer and then logs off. When B logs on, they see A's trade offer and can accept it while A is offline. When A logs back in, they get a notification that B accepted/rejected their trade offer.
How to prevent people from spamming offers to everyone:
Whitelist system where B has to enter the username of A into a list before A can send B a trade offer. A can only then send B a trade offer (even if A doesn't whitelist B).
why?
imagine trying to trade with someone who's on the other side of the planet, or you're managing a shop and you don't have time to meet up with customers.
also since this only matters to discord users, maybe add a disclaimer that this is discord exclusive

or 
