If you deal with a seller whom provides you with a bad product you have several options as long as you paid with paypal or a credit card. Keep in mind, if you , the buyer, order the incorrect part or did not read the whole auction, it is not the sellers fault your not happy. You can then contact the seller and see if they are willing to work with you. Most sellers, if you admit you made a mistake and are willing to accept responsibility for your mistake by paying for freight and any incurred fees, will take an item back within a reasonable time frame. Often this is solely based on the buyers attitude, timeliness and sincerity. Start with an attitude and expect to get one back. Especially if you made the mistake.
Now if you bought an item and it was incorrect or not as described, and you paid with paypal, after contacting the seller and they are able to or willing to resolve the issue, contact paypal within 45 days of the sale and file a dispute. Use the option for "merchandise not as described" it will give you the best result. The buyer will have to provide a replacement or allow a return for a full refund. You will have to ship the item back at your expense to the address paypal tells you to but at least you will get your purchase price of the item and the original freight back. Better than nothing. Do not expect to recover the freight you paid to return the item. You could ask the seller for it in exchange for positive feedback. Ebays feedback extortion rules clearly list that "A buyer says he or she will leave negative Feedback or low detailed seller ratings (DSRs) unless the seller accepts a return and pays for return shipping." is NOT considered feedback extortion and is allowed. With ebays fees on sellers ties directly to their feedback rating this is a good tool to hang over a seller.
If you pair with a credit card. Call the toll free number on the back of your card and file a "chargeback". Your card issuing bank will take the money from the seller (or paypal if you used your card through them) and place it in escrow. The seller will have 14-30 days depending on the bank to prove why they should have your money. Again, state "item not as described" for your reason. Most banks will ask what you want/expect for a resolution. In this area you should state "a prepaid return label and full refund or the correct intact item as offered in the auction". Most sellers will not issue a prepaid label or even respond to the chargeback. In both cases you keep the item and get your money back. If they want their product back the bank will make them issue the prepaid return label.
In either the paypal or credit card case DO NOT just ship the part back. Keep ALL of your communications and do them in writing only. Do not make phone calls to the seller. The spoken word means nothing to paypal or the credit card companies. Its a he said/she said issue and if its not in writing it did not happen as far as they are concerned.
Most of all, keep in mind that most sellers are not evil and trying to steal your money. Most of them will work with you to make you happy within reason. So be polite, friendly and give them a chance to make it right before you go off the handle and scream and yell. We all make mistakes, both sellers and buyers, and its easy to resolve if every one admits to their mistakes and work on making it correct.
I hope these instruction will help any buyers whom get stuck with those few bad sellers that are out there.