I'll echo what everyone has said about pictures. Lots of good quality pictures, and be honest about any damage or blemishes. Beyond that I also think it's helpful for a seller to say why they are sellling something such as a bike part. Did you upgrade? Wrong size or color? Got it as a gift and don't want or need it? If you include those details people are less skeptical. This doesn't apply if you are selling collectibles or overstock type stuff, but if you were selling for instance a wheelset or saddle for a bike, people will want to know why you don't want it any more. Make sure you list it in the right catagory, and don't use the misleading, attractive titles, like so:

"Specialized seat post. Not Trek, Giant, or Orbea"

People do that to get hits on searches for Trek, Giant, and Orbea, and it's deceptive. I absolutely won't deal with someone that does that crap.

After the sale, whether you are a buyer or seller, be quick with communications, payments or shipping, and feedback. Feedback is your credibility, you want it spotless and positive.

As a buyer, you should have some idea what the item is worth on the market, and then decide what it's worth to you. Put your highest bid in and be done with it. You'll beat a sniper that way, unless they are going higher than what you are willing to pay, and save yourself a lot of time and worry. Remember, the price will only go that high if someone drives it that high, putting in say $25 as your high bid for a $12 item will only take it to $12.50, but will keep up with the bids automatically up to your price. Even if you don't want to do that, stand on your high price. It's easy to get into a bidding war and take an item well above it's value.