Look for good seller feedback, and the more the better. If there is negative feedback, see how recently and see if/how it was resolved.
Read the item descriptions carefully. If the description doesn’t say something is included (like a charging cable for a cell phone or an HDMI cable for a DVD player), assume it’s not. Study the photos carefully, good sellers will provide photos that show all defects on an item. If you don’t study the photos carefully then it’s your own fault.
Note where the seller is located and their shipping policies and price. Also note their return policies, I normally assume no returns but many sellers will accept returns.
As far as being a good buyer, have your PayPal in order with a confirmed shipping address. Don’t try to have the seller ship to a different address, that causes red tape and PayPal and Ebay do not like that and the seller will be blamed if anything goes wrong. So don’t ask them to do it. Don’t try to haggle or change terms. If you’re not happy with the item and terms as listed in the auction, then don’t click Bid or Buy it Now.
Wait until you get the item and confirm it is as expected before leaving feedback. If there is something wrong give the seller a chance to make it right before leaving negative feedback. Most sellers will leave you good feedback after you leave them good feedback.