Going to make a "New Trader Guide" and I'll need assistance

Recently, I had some dealings with people new to the trading community as a whole and let me say they were pretty interesting experiences.
Heck, I would have been a bumbling idiot my first day here if I hadn't been using tf2outpost for ages beforehand
Since I can't think of everything, I'm turning to the community for some support

What I've got now:
Be cautious
Keys can't be used more than once
Dota 2 keys are NOT game keys
keys=codes
some stuff is region-locked