Always check a user's feedback as a buyer AND a seller; you can save yourself trouble later. I go to toolhaus.com (link below) to see if they've had feedback withdrawn - - you can search for negative & neutral feedback there without sorting through hundreds and hundreds of feedbacks. A VERY useful and time-saving tool!
Read the entire listing carefully. Terms of sale are often listed on the bottom of a listing, so be sure you read EVERYTHING so that there are no surprises. It's especially important to note what types of payment are accepted, how much shipping will be (and by what method), and what the return policy is.
Be courteous in your communication with others; attitude goes a long way in a smooth and successfull transaction.
ASK QUESTIONS! Most sellers are more than happy to share additional information that is not in the listing or clarify what's in the listing. If you don't ask, you won't know.
Be on time; Respond to emails & messages, pay for your items, mail your items, etc. promptly and without delay. If you know you will be held up for some reason, let the other party know the reason why before hand so that they don't think you simply forgot. Any reputable buyer or seller won't mind waiting a bit so long as you keep in touch with them and keep them informed as to what's going on.
Research items you are looking to buy or sell. See how much they have sold for recently. Compare prices, shipping costs, feedback ratings of sellers, etc. It's often worth spending a tiny bit more money to buy from a trusted seller with a high feedback rating percentage.
Search Smart: Use the advanced search engine and learn how to use the right key words to search smart. Search descriptions as well - - often it's very hard for a seller to fit all the important information in the listing title. Try different terms that mean the same thing (i.e. bathing suit = swim wear, etc.). Familiarize yourself with the various abbreviations that are often used in listing titles such as:
(B)NIB = (Brand) New in Box
NR = No Reserve
NWOT= New Without Tag
(V)HTF = (Very) Hard to Find
XC = Excellent Condition
EUC= Excellent Used Condition
NRFB= Never Removed From Box
NWT= New With Tags
BIN= Buy It Now
S&H = Shipping & Handling
GU= Gently Used
MIB = Mint in Box
OOP = Out of Print
VF = Very Fine
GBA= Gameboy advance
NDS= Nintendo DS
Don't wait until the last second to bid! If two bids of the same amount end up as the the highest bid, the person who placed the bid FIRST will win the item. Many people use bid sniping software that submits their bid automatically within the last 10 seconds of the auction, so just because it APPEARS that you are winning an auction, you can still lose it at the very end. Make sure you place your bid for the MAXIMUM amount that you are willing to pay.