When you buy something on eBay, you buy it ON eBay, not FROM eBay. eBay is made up of millions of different businesses, small and large. Your deal is between you and the seller. eBay will only get involved in extreme circumstances.

If you bid and win, pay promptly. Also, think twice before you bid. eBay considers your bid a binding contract.
If you have questions about an item, ask them before you bid. After you win is not the time to get information about the shipping costs. If you don't like the seller’s response, do not bid. Each seller gets to set their own Terms of Service, within eBay guidelines.

Use eBay’s My Messages to communicate. Always include the auction number on any mailed payments, as well as your name and address. I have received payments with no auction ID or name on them and had to figure out by process of elimination what was being paid for.

Know your rights and responsibilities. If you are frustrated with your seller, check their contact information. You have the right to get a phone number for someone you are in a transaction with. Go to Advanced Search, and look in the left navigation bar for the link.

The seller does not have the right to change the auction terms after the auction closes (charge higher shipping, refuse to sell the item because it "sold too low", etc.). If they try to do so, you can report them as a non-performing seller. If you see listings that violate eBay guidelines, report them--there is a link at the bottom of each auction. eBay has always counted on the community to regulate itself.

Let your seller know you are happy (or not) when you receive your item. Most sellers don't consider the transaction over until the buyer has let them know that they have received their item and are satisfied. If you aren't happy, contact your seller and see if they will work it out. Negatives should be your last resort, not your first.

Leave positive feedback when you receive your item and are happy. This will greatly improve your odds of getting good feedback.

Under Federal Mail Order Law, the seller has the responsibility to get you the goods or your money back within 30 days. If you have a slow-shipping seller, and you don't receive what you paid for, use your tools. If you paid via Paypal, you can open a dispute. If you paid other ways, you can open a case with eBay for Item Not as Described. If all else fails, you can register complaints with the police in the local jurisdiction of the seller, their state Better Business Bureau, or even the FBI. Most sellers are ethical and are trying to do the right thing.

On the other hand, don't try to scam the seller. Many sellers use delivery confirmation and other tools to make sure packages are delivered. Don't tell him it didn't get there when he has the scan that tells the delivery was 3 days ago.

Choose your seller wisely. Don't just look at feedback percentages. Dig a little deeper before hitting the BID button. A little research up front can save you a lot of grief down the road.