There's been a wealth of great suggestions in the posts above. Since I tend to collect pipes that have been unavailable for decades, eBay has been a primary source for me. Over the years I have my set own guidelines for buying on eBay and some specifically for buying pipes. Here are some of the less esoteric ones:
1. Follow some auctions to get to know what general market prices are for what you are seeking before you start bidding. Yeah, you'll let some opportunities pass you by, but you know what? There are always more coming and you'll have a better idea surrounding a fair value for your idée fixe. You might be less inclined to overbid.
2. Ask the seller about any questions regarding the pipe that interests you. If the seller isn't responsive, avoid that auction.
3. Ask for better pictures if you're not satisfied with the ones that are posted. With pipes, I always want to see the business portions of the pipe - the rim, the slot, close ups of the bit, if possible, the chamber. If I don't see those, I don't bid. Period.
4. Use your eyes. Look critically at the pictures of the pipe. Verbal descriptions can be very inaccurate. If you're good at "reading" pictures, like I am, you can occasionally get some great deals on badly presented pipes. But if you're not good at it, you can get a real stinker.
5. Don't put too much trust into seller ratings. Some of the biggest snakes I've encountered on eBay had 100% approval ratings.
6. Remember that many sellers don't really have a clue about the pipe that they're selling. They bought it at a garage sale and they're not necessarily trying to be misleading, they just don't know. With eBay, it's YOUR job to know what you're doing.
7. Do not get into a bidding war. That's great for the seller and bad for you. Check your ego and testosterone at the door. There will almost always be another opportunity.
8. Never place a financially stratospheric snipe at the end. Believe it or not, you're not the only genius with this idea. That's how $600 pipes go for $3000.
9. Don't get emotionally attached to "winning". That's the hook that eBay uses to get people to overbid.
10. Educate yourself to be able to see the signs that a pipe has been smoked too hot, otherwise abused, or had extensive plastic surgery. They've visible for those who have learned to recognize the signs.
11. Have fun!