Trading:
*Keep an updated list of cards you would like to obtain ("Want list")
*Have a list of "Haves" -- IMO this isn't as important as the "Want List" because if two people both have updated "Want Lists" then they can just look at each other's list to see if there's potential for a trade.
*Use a price guide to make fair trades--keep in mind that some cards are valuable due to being powerful cards used in a lot of decks (Grapes, Mayhem), some cards are valuable due to scarcity (Nicolas of Antioch) and some cards are both (King of Tyrus WA and Daniel Promo).
*Follow through on your trades and if you cannot, be honest and try to make it right for the other person. I can remember a time when I thought I had a copy of a card I agreed to trade but then realized I didn't (or I didn't have a copy in good condition). I asked the other person if they wanted to make a different trade, if they wanted to cancel the trade or (in the case of the non-mint card) if they wanted me to add more to my end of the trade.
Selling:
*Always be specific with a card's condition--different people have different definitions of mint/near-mint so be detailed.
*Communicate (particularly if there is a delay in shipping--most people are understanding, but they like to know what is going on)
