With 2.5 years experience selling and buying on eBay, I know the pitfalls of buying items--especially consumer electronics. As I recommend to my customers who buy mobile devices, choose the cellular carrier FIRST, based on J.D. Powers and other  impartial sources, then choose a service plan and phone.  The same applies to choosing a seller. Choose an excellent seller who's selling what you want, then buy . Here are a few recommendations:

• Avoid the temptation to buy at the lowest price. Red flags are seller feedback below 98% or a seller who recently began selling on eBay. These are not knock-out factors; just make sure you've checked.
• Beware of sellers whose Buy it Now price is significantly lower than other sellers. Trouble lurks.
• New vs. used. Beware of sellers who claim items are new when they're used. If a listing indicates the seller  "tried out" an electronic device to see if it works or for "testing," the item is used 
• Click several items under the seller's feedback ratings. Determine if the seller specializes in the type of item you're buying. If the seller has expert knowledge about an item category, the seller can better answer your questions before you bid or buy and help you after the sale should something go wrong.
• Click on a couple of negative feedback sales.  Sellers aren't perfect and neither are buyers. Review how the seller responded to negative feedback. Professional sellers go the extra mile to resolve negative feedback; non-professionals react to negative feedback by accusing the buyer of stupidity or not reading the listing or, worse yet, name calling. Avoid these sellers like the plague.
• Click on the bid list. Experienced buyers on eBay watch who bids. If most of the bids come from sellers with 50 or more transactions, you should feel more confident about bidding on the item yourself.
• No or poor communications between buyers and sellers. If I chose ONE action to take BEFORE the sale, it's sending questions through eBay email to sellers and reviewing responses. Large Powersellers may offer lower prices; however, if a seller doesn't respond to your questions before the sale, poor communications may occur after the sale. This isn't always true, but I've had enough negative experiences with large sellers who fail this test. In  most cases, I avoid non-responsive sellers.
• Returns. Avoid sellers whose return policy is "no returns, all sales final." Pursue sellers who offer immediate returns if your item arrives DOA or not as depicted. A three day return in my book is minimum for returns. Also watch the "re-stocking" fee sellers. If you're item arrives not as described and the seller accepts a return with a 15% re-stocking fee, you lose money on shipping charges and the re-stocking fee.
• PayPal Buyer Protection. Get ready for a one month wait. PayPal can help you with non-responsive sellers who either failed to ship you an item or sent you a "not as described" item.  However, the proof, except for non-receipt, is on your shoulders. I've lost PayPal disputes when an item was clearly used, but PayPal required a third party to write a letter supporting my claim. PayPal wouldn't even accept a manufacturer's invoice to prove my case. TRY to work things out with the seller FIRST. Then, if that fails, using PayPal, you can file a claim to force the seller to respond, then escalate the claim to a dispute permitting PayPal to resolve the situation. Again, get prepared for a long wait.

If you follow the above steps, you're more likely to have a positive buying experience on eBay by buying from excellent sellers.
Brian