The best thing to do is list what you have available to trade. It's a tedious process but it is very helpful. And don't think that something is too common to list- there are people collecting every set, and when I listed my tradelist, the set I considered the most common and figured nobody would want was the first set that people asked for.
Shipping is big. Most people don't like getting a PWE- Plain White Envelope- because there is a long history of the cards getting bent up and mangled doing that. The preferred method is in a padded mailer, with cards in toploaders and team bags. If multiple cards putting them inside a team bag with toploaders on the end is also acceptible.
Try to ship as quick as possible. If you can't tell the other person. I can only mail on Saturdays as a general rule but I tell every trade partner and not one has had an issue with that yet.
The main thing is to finish a trade if you agree to it. Don't take people's cards and run. Real collectors aren't going to do that, so it shoudnt need to be said. But I have seen, over the years, enough people not following through that it does need to be said.
If you don't like an offer, tell the other person that it's not going to work or maybe something else can be tried. Don't just ignore them. It's really frustrating when you are trying to make a trade or purchase and the other person doesn't bother to reply.
Those are pretty much the basic of basics. Hope that it helps!
VERY slow trading due to health problems. Not transferrable so safe to trade with, just moving is painful and can't always access the cards.
Cardboard History My COMC
New Collection Website: Cardboard History Gallery (Still under construction)
Tips on how to make your scans look like the card does in hand (No more washed out, fuzzy scans!):