Export: As I said, if it's Steam to Steam trading, that can be done in-client,
You do know the concept of a "Steam Key", right? It's just a string of numbers, that the only way to check is to try to redeem it, with one of the following outcomes.
1) You redeem it, and game is added to your games list
2) You try to redeem it, but you already have the game, thus key remains valid
3) You are notified that the key has already being used
4) You are notified that the key isn't valid for your region
5) The key is invalid.
While trading of steam keys can be done in-client, it's no different from doing it through any other messaging system, be that PMs, e-mails, text messages or even plain snail mail.
And as for a "Trusted Traders" list, there is one in the classifieds thread, usually after each offer. It usually reads something like "previous trades with: @@@, XXX, YYY"