Do your research. Don't buy something unless you know the brand and know how it fits you. If possible, try it on or (if it's a bag) visit it in person. I rarely shop randomly on eBay, meaning, I almost never go search for "t-strap shoe" because I'm going to get all manner of horrible strippery pleather things. Know what you want and know the brands you want. If there's a particular hard-to-find item you're looking for, set up an alert so eBay will email you when one is listed.
Everything is Faked. If the item is designer, know what details to look for or find someone who does. There are great online resources for bags, jeans, shoes, you name it. The Purse Forum is a godsend for designer bag fiends. Just remember: Everything is faked. I cannot say that enough: Everything. Is. Faked. This is especially true of designer bags and jeans. Shoes don't get faked as often, but it does happen.
Keyword tricks. Lots of people stick the word "Anthropologie" in the listing even if the item didn't come from Anthro. (This is against eBay rules, just FYI.) This goes back to knowing your brands and knowing what you're looking for. If there's no clear description/photo, email the seller and ask. If you think to yourself, "Wow, I didn't know Anthro carried that brand!" it's probably because they don't.
Photos are everything. If the seller photos suck, email and ask for better ones especially if the item is used. Don't be afraid to explain the concept of the macro button for close-ups. If the item looks trashed in the photos, trust that it'll probably be worse in person. Conversely, if the seller provides good photos and describes every tiny scratch and ding, that's a good sign! It means your seller is diligent about the item and not trying to hide anything. You'll probably find that the item is better than it looks. Also! I almost never bid on an auction that only shows stock photos. I need to see the exact item I'm buying.
Ask questions. Email about anything not addressed: smoke-free home? Any undisclosed damage? Has the item been dry-cleaned? If the seller is rude or doesn't respond at all, move on. Sometimes I'll ask a question just to see if I get an answer, especially if the seller has low feedback.
Check the feedback. It's pretty much all you have. I like using Toolhaus to suss out all the negative feedback but be reasonable: A seller who has 50 negs out of 12,000 total feedback is probably okay. (Unless the 50 negs talk about the items being fakes. Then you run.)
Best Offer is awesome. If the seller has a Best Offer option, go ahead and make an offer, but don't insult them. I usually assume a 10-20% discount off the Buy-It-Now price.
Be a repeat customer. If you find a seller who sells something you love and you had a good experience, add them to your Saved Seller list and revisit from time to time.
Be a sniper. For the love of God, do not get into a bidding war. You are just driving the price up and you'll probably lose. DO NOT bid early and often. Bid last. Sniper software is your friend and I recommend Just Snipe. It's sneaky but it works really well.
Take a risk. If you love a pair of boots you saw at Nordstrom last year but never even tried them on, do a little research via online retailers. Anthro now has website reviews, so does Banana Republic and Zappos. Read the reviews and try to get an idea if they'll work for you, then go for it. If they don't fit? Back on eBay they go.
My most recent "take a risk" eBay purchase was a pair of slightly trashed black Frye t-straps. The seller photos/description were terrible but I love my pewter pair (and they're on their last legs) so I snapped these up for a very low price knowing I was taking a risk. They arrived looking, well, pretty trashed:
(What did she do to you, poor shoes?)
But, all the damage is on the outside and easily repairable. I obviously need to get them re-heeled and polished up, but the insides were in really good condition and the soles weren't terribly worn. It was a risk, sure, but one that will hopefully pay off. I'll do an "after" post when I get them back from my shoe guy.
So! What have you bought on eBay lately? Do tell.