Don't bid unless you intend on purchasing the item.
Know how much the item is worth.
Know how much you can afford.
DON'T BID ON TOO MANY THINGS AT THE SAME TIME!!! Chances are, you'll win all of your bids...and be broke.
Communicate with the seller immediately. Answer all emails.
ALWAYS check the seller's feedback BEFORE you place a bid.
Wait until the last half hour of the auction to bid.
Send payment promptly.
Tips for selling on eBay:
Don't post the item unless you intend to sell it.
Don't start your bidding price too high -- you won't get any bids.
Do some research before you sell your item. You can search the completed auctions and get an idea of what the going price is for the item you want to sell. Then, you can decide if you want to list your item.
Don't rely too heavily on the price guides when selling on eBay. What something is worth is whatever someone is willing to pay for it. If you think you can get more (book) value out of a flea market or an antique store, then try selling there. I've seen many items on eBay go for far less than what the price guides say they are worth.
If you can include a picture, by all means do so. I've noticed that items sell for a higher price when folks can see what they're buying.
As soon as the auction ends, contact the bidder and ask that they send a return email to acknowledge the sale.
Always list your item accurately...any flaws in the item, chips or anything that you think YOU would want to know if you were buying the item, you should list in the description.
DO accept personal checks. All you have to do is hold the merchandise until the check clears. I've noticed a lot of merchandise not sell because the seller only accepts money orders.
If you want a certain price for your item, then list it with a reserve price.
If possible, find out the shipping charge before you list the item and list it in the description.
Pack your item WELL with a lot of styrofoam peanuts, bubble wrap or crunched up newspaper...anything that will protect it from being smashed by shipping/rough handling.
UPS and if your item sells for enough money, the US post office give tracking numbers. Provide these to your buyer.
Always insure the item.
Have fun and hopefully make a little (or a lot) of money.