I am a relative newcomer to ebay. Until I became interested in estate pipes, I really had no use for it..
But after several purchases, I've learned a thing or two. First thing I learned is that ebay can be a great resource and I am not afraid to buy from it, but I've also been burned.
Case in point, I purchased 2 Peterson estates from an ebay-rated "Top Reseller" with a perfect rating (100%). The photos on these 2 pipes showed absolutely no fills, and yet there were fills galore. I work in photoshop and these photos were clearly tampered with. Both were returned.
So the lesson here is, don't trust anyone, even with a perfect score. And always make sure the reseller accepts returns. Never, ever buy from someone who won't. I've seen quite a few that say no returns or exchanges—always double-check.
Second lesson I learned is, read the description VERY carefully. Read it 2 or 3 times even. Important info is often hidden in a paragraph of text.
Three: Check the dimensions of the pipe very carefully. Get a ruler and see how big the pipe will really be. If you have a pipe that's the same size, hold it in your hand to really gauge the size. Pipes look a lot bigger in photos. Also, with every single estate I've purchased, the stated size was not accurate - usually the pipe is just a little smaller - close enough where they can claim an error (one time it was way off, and this wasn't from an ebayer, but from a respected sponsor of this site! But I believe it was an honest mistake on their part and mistakes happen).
Four: Check where the reseller is located. I got really excited about one pipe, until I realized they were in S. Africa and shipping was $20. On that note, always check the shipping costs too.
Fifth, bidding can be contagious. One can easily get caught up in the frenzy of a bid and quickly go higher than makes sense. I've seen plain old used pipes (with no collector value) sell for more than a brand new one!
From reading the above you might be dissuaded from using online resellers or ebay, but for me, having the plethora of choices outweighs the problems. And as long as I can get a refund, I will continue to buy pipes online.