1. Look at pictures in ad. Read description, but don't trust it. Bad pictures=high risk.
2. If pictures are unclear, ask for a picture of the part you worry about. If the seller is unwilling, be careful.
3. Ask questions if ad is unclear. If the seller does not appear to know what he is talking about or being evasive, thats a warning sign.
(Save all communication. You can delete it later, if all goes well).
4. Always, always, always purchase with Paypal the proper way. If the seller insists on using the "gift" option, there are two possibilities: He's a scammer trying to f -you or he is a cheap son of a ..who's probably going to send the item the cheapest way possible, wrapped in old newspapers and snotty hankys. If you are still really hot for the item and the guy won't budge, pay the fees yourself. The paypal protection is worth the cost. And for heavens sake: use the Ebay system, otherwise you are not protected. Don't make shady deals.
5. When item arrives carefully inspect packaging (with costly items). If something is seriously damaged or tampered with deal with it when the item is delivered. If you have allready received it, and it is too late, take pictures of the package(s).
6. When item is unwrapped, carefully inspect it. Take pictures if something is missing/damaged.
7. If something is off, you can try to communicate with the seller, but be professional and civil. Make your complaint known in a matter-of-fact way. A gentle touch will often get you more than coming out, guns blazing, and sending crazy emails will make you look bad and unreasonable.
8. Feedback should be left the next day, so as to give you time to cool off and think.
9. Having documented everything and followed the correct procedure, you should be in a very good position in case the deal goes bad.