if you guys are so serious into trading you should probably look into having some honest persons as a moderator in your trades
i.e in case of steam gifts both of people send the gifts to the trusted moderators and he completes the deal or else trade is cancelled, gifts returned back
in the case of keys well its hard to moderate that.
I am trying to provide an automated moderator. People are just not test running it and feeding back.
If people would use the site here: http://williampegg.net/Trade
then I could get some feedback, improve it, and ultimately produce something that everyone trusts. Right now it's very raw, and requires the users to keep clicking the refresh link, but it's fundamentally in place, will validate GOG codes, and I think it works. It requires a log in, but that's a simple registration, no details required.
I just need people to test it.
I'm following this work of yours . :)