I’m a self-proclaimed Ebay-holic. I have both bought and sold items on Ebay. In all cases, I believe that you have to be fairly savvy to get what you want as either buyer or seller.
Some general guidelines that I ALWAYS follow:
Buying: Know exactly what you want. If you want new DA cranks in 172.5, don't settle for "slightly used," especially when a picture clearly shows a crankset that has the entire DA logo rubbed off and bright silver teeth on the chainrings. Don't buy a frameset hoping it fits. Get fitted for it at a brick-and-mortar shop before trying to aquire it. Bottom line: don't HOPE something is what you want/need. KNOW it.
Know EXACTLY what you want to pay for it. If you want to pay less than $120 for those "brand new" DA 172.5 cranks, then put that in as your max bid. If you lose that auction just keep trying. Sooner or later you'll get the deal you want. I doubt that whatever you want/need is going to make or break your season if you don't get it RIGHT NOW. If it is, you should be buying at the LBS or an online shop.
Don't bother with any auction without pictures. While most of these are probably honest people just trying to unload excess gear, I don't trust anything I can't at least see. And don't fall for someone stealing the pic off the company's website to post. Make them show you a picture of the actual product they are selling. People selling Cervelos are notorious for putting Gerard's picture on their auction because it looks cool. Trouble is, the bike the seller is selling doesn't come with the 909's. If you can't help yourself and entertain looking at auctions without pictures, make them send you one. If not, well, its your $$. Another thing about pictures is that you should look for pictures of any damage to the product. Ask the seller if it has any marks/dings. Make them show you.
As a buyer, don't bid until the last part of the auction. Many contend that this is sniper bidding. If I'm the seller, sure this annoys me and I'll rail against the ethics of it. But if I'm the buyer, why start bidding on day 1 of a 10 day auction. That will only drive up the bidding sooner. Lay in wait while the previous bidder remains content with his $110 DA crankset. 5 minutes before the end of the auction bid your max of $120. If you win, good for you. If not, see above.
Always try to communicate with the seller prior to bidding and throughout the auction process. The more you communicate, the better you'll know how legitimate the seller is. If they're evasive or don't want to comm, don't bother. See below on patience.
ALWAYS, ALWAYS, ALWAYS buy with a credit card that has buyer insurance. I violated this only once, and let me tell you, hanging out there for $800 on a money order was tense, even though I had great comm with the seller and got the bike exactly as advertised. I won't do it again. At least with a credit card you can get the power of the CC company's lawyers behind you if you get had by a scumbag.
The number one hint on buying is PATIENCE young Jedi. I've bid on about 12 *New* Polar s710's now and lost every one. Sooner or later I'll get one at the price I'm willing to pay. If not, well, I'll keep using my s610. Its ok to be a cheapskate. Just be a smart one.
Selling: Accurately list your item. Don't just sell a "road bike." Sell a "2002 Specialized Allez Comp, 54cm (Giant, Cervelo)" As a buyer I am always annoyed at and I ALWAYS skip auctions that don't tell me what size the bike is. You'd be amazed how many people do that. Can I ask the seller? Sure. But if they don't have the wits to put it in there in the first place, chances are there is a lot more they aren't telling you. Why did I put (Giant, Cervelo) in the line? Simple. People searching for Giant TCRs and Cervelo Soloists, two other similar bikes with sloping geometry will see my auction too. Maybe my auction will convince them to buy mine. Its all about exposure. The more people that see your auction, the more people will bid. More bidding=more $$.
Always put accurate pictures of the item on the auction. The slight cost is more than offset by the increased selling price. As a buyer, I won't even entertain auctions without pictures. Why should I expect others to do the same? If your item is especially striking in appearance, as a recent P3 "show bike" was, you'll stimulate a lot more bidding. Heck, I just bought $5000 worth of Litespeed and was more than a little tempted by the P3. If the item has damage, say a ding in the top tube, put a photo of it on the auction. If the damage is major enough to cause people to stray from your auction, you shouldn't be selling it anyway (A recent thread concerning out of true Rev-Xs comes to mind). Give that sort of stuff to a local junior or tri-newbie. The money you might make on it at auction isn't worth the hassle and will definitely not ingratiate you with the buyer, no matter how honest you are.
Always set a reserve. As in buying, know what you want/need to get for the item and don't compromise. This rule is somewhat flexible. You aren't going to get $1000 for a "1994 Specialized Allez Pro 55cm Ultegra, Steel frame," no matter how many upgraded bits are on it, unless you have a brand new set of 404s mounted on it and are selling them with the bike. That was a real auction of mine. I set the reserve at $300 (I paid $1000 new) and hoped for $500. I got $350. That was exactly what my LBS was willing to give me on trade-in if I bought my new bike in the store. No harm, no foul.
Charge the buyer shipping costs, unless you're selling one of those $5000+ wonderjobs and got at least what you wanted for it. You can get estimates from any Mailboxes.etc just by taking the prepacked item in.
Give the buyer the most options in paying for the item. Although I advocate CC only in buying, give your buyer the option to pay by check, money order, etc. Reality is many buyers are younger kids without the CC option. As long as you get positive control of the total $$ before shipping, no hassle. An addition to this is to advertise in your auction that you will not sell to buyers with negative feedback. This just falls in the category of setting yourself up for success.
This is quite a long post, but amounts to the majority of my tenets of auction buying and selling. I may still get had and, given the amount of transactions I do on auction sites, the odds continue to increase that I will. But these precautions will prevent probably 99.9% of the scumbags out there from getting their hands on my hard earned $$. Its a conservative strategy and hedges a lot of bets, but then again thats what these auctions essentially are: gambling.