Online auctions-- I started looking at them recently after looking at some of my Classic Traveller stuff and deciding I'd like to complete the set. Overall, this has been a nice and somewhat safe process. As far as I can tell right now I have not been ripped off, but the temptations in online auctions are huge-- you can literally find enough bargains to go bankrupt over. In past days I have almost completed my collection of Classic Traveller, found my missing Shatterzone book, picked up every other 2300AD book except one, and found a bountiful assortment of Yaquinto Games (and remembered in part why I sold them in the first place).
The first rule of thumb, there are scam artists out there. Protect yourself! One of the ways that you can do this personally is to always send money orders by registered mail. By accepting the letter and signing the receipt if the person doesn't send you the merchandise or it is obviously not what you bid on-- then they have committed a crime. Personally, I did this until I got more comfortable, now I only send money orders if the seller wants one, and I don't mess with registering the mail. I figure if I'm ripped off, I'm ripped off-- the crime is on someone else's conscience and I'm not going to worry about it.
On ebay you have these wonderful lists of recommendations to look at, ands that is a help. You can also find out where the seller lives, though by asking the seller also finds out where you live as well, and email them. I've had questions in a couple of auctions that an email exchange really helped with. Sometimes someone sells multiple copies of the same item in different auctions and it can really send up some red flags to see the same merchandise from someone being auctioned close together.
The second rule of thumb, be patient! If I had been more patient I would have saved a significant amount of money in my hunt for Traveller stuff. It turns out the Marc Miller is running a site at http://members.aol.com/SFRPG/T13.html, and he is selling some Classic Traveller, and 2300AD stuff at a significant discount. Other sites also do the same. I've searched through sites like Dragon Trove (dragontrove.com), Titan Games (titan-games.com), and Sword Worlder's (http://mbr-scifi.neotown.com/freetrav/) looking for stuff, and I would use these various prices as to how I would bid an auction. Unfortunately I bid too high on some, when I could have bought something outright much cheaper.
Patience is also a key when you see an item that you've always wanted coming up for a bid, and you have to have it. I bid $100 in the last five minutes of an auction for the old board game Fifth Frontier War for Traveller from GDW. Thank God, I was outbid by someone else. The bids stood at $40 when I put in my nuke bid, but there was no way I could have put together a $100 cash outlay that easily. I was jeopardizing my own family's financial health because of a game I wanted from mt childhood. The next week Fifth Frontier War came up for auction again-- I didn't bid this time because I gave up purchasing games for Lent. I was also suspicious because the new auction came so close on the heels of the last auction, and it was from a first time seller. Would this person be tempted to try to rip someone else off of $100? I don't know, but it certainly raised some red flags for me, and I don't believe that the bidding got as high in the second round as it did in the first. Patience can also pay off in that other people will be selling the same thing later, and you might just be able to get it cheaper.
The third rule of thumb, speed can be a killer. I have been in auctions going, "Oh Yeah. I'm bringing that home." Only to get an email a minute before the auction closed saying that I'd been outbid! It was too late to do anything. Sometimes you can fight someone else's speed with patience of your own. By not declaring a bid early you can sometimes win with a lowball bid at the end, or you can try to nuke everyone with a high bid right at the end-- just remember, you are legally required to pony up for your nuke bids.
The fourth rule of thumb, you don't have to have it. Don't get into bidding wars. Let someone else pay too much. Be patient-- these games have a way of reappearing (except for Runequest 2nd edition and Gurps Old West).
The fifth rule of thumb, out of print often means one of three things-- mold, smoke, or mothballs. I have been astounded at the condition of some of the items I have received. It might look like a mint condition book on the outside, but it can stink to high heaven on the inside. Only Titan Games sent some things bagged and boarded. I got some other stuff that smelled like old stale smoke-- thank goodness it hadn't been soaked in bong water-- and some other stuff that smelled like mold and mildew. Smoke comes from being used in games, and mold and mildew from being locked in a closet for years. I have received one game that I've had airing for two weeks because it smells so strongly of mothballs. Typically when we ask about stuff we wonder what it looks like, but after twenty plus years that old Traveller stuff can have some powerful funk in it.
These are just a few observations from a Screaming Jackass. Out Of Print stuff can be wonderful. You don't have to worry about new rules sets, or expansions to buy-- but other people can like that classic stuff as well. Be patient, and eventually you will find what you are looking for.