I've sold a few things on Ebay (40+ positive feedbacks, not that many , but still enough to have opinions to share).
First off, in the item description, give the impression that you give a shit about whatever you're selling, so do post good pictures. That means enough lighting, use a tripod, and (can't emphasize this enough) make sure the item's sharp in focus. If the item has observable defects/blemishes, document and show those too.
Side note: If you describe a product as NEW and the item turns out to have flaws, the buyer has a legit claim to get his/her money back. Describe it as LIKE NEW or EXCELLENT+ or EXCELLENT+++++++ and they'll be hard-pressed to bitch you out.
Answer questions/emails truthfully and respectfully, but don't make yourself look desperate. By that, I mean even if you check your email every 10 minutes, take time to answer it (rule of thumb: more than a couple hours but no longer than a few days). As long as you reply eventually, the buyer cannot accuse you of being a bad communicator. That way, buyers will know that you have better things to do than being their servant, and they won't feel compelled to manipulate you. This is what I mean by manipulation:
- negotiating fees/shipping *after* the auction closes. "How about we split the Paypal fee?" "Screw you!" You've set the fees in writing, they agreed to it by placing a bid, and they negotiate after the fact? I hate this so much.
- senseless claims. If they say the item has flaws that you know doesn't exist, and they ask for money back for repair cost, just ask them to return it for a refund excluding shipping costs.
Only leave a feedback after the buyer receives the item, approves it, and leave you a positive feedback. Since most buyers have less # of feedbacks than sellers, even the fear of retaliation will keep their behavior in check.
No picture to show, but here's a bad academia joke I witnessed last week:
Student: "Professor, I heard a rumor that you're actually just a big softie who likes to feed squirrels, is that true?"
Professor: "..... I ... feed ... squirrels? ..... YES ! Wanna know what I feed to squirrels?"
P: "Little pieces of grad students."