1. I tend not to buy anything from auction; I’m generally a Buy It Now only shopper. Buy It Now works essentially like any other online shopping – the price is set, and you can agree to it. Auctions stress me out, especially since the “maximum bid” feature was introduced.
2. Given that I pretty exclusively buy from Buy It Now selections, the seller rating matters a lot. [I’m sure it matters a lot with auction items, too.] I don’t buy from any seller with a rating below 99%. This is not as steep a standard as you might think.
3. Some Buy It Now products have a “make offer” button below the price. ALWAYS USE THIS. Never ignore it. It essentially says that the seller is willing to agree to a lower price than s/he set for Buy It Now. Even if you think the Buy It Now price is fair, you might as well try to negotiate a lower price. This is eBay, not philanthropy.
4. If you’re asking “Why is this on eBay for $20 when it costs $30 from the merchant?” it is a GOOD QUESTION, and one you should constantly be asking yourself. [N.B. this applies more specifically to new products at Buy It Now prices. Used products, such as the above purse, and auctions have different rules.] You do NOT want to get caught up in someone selling stolen property. Generally the most reliable products are the ones that are only a few dollars (or percentage points) below the price you’d find in-store, because those sellers probably got the products in bulk or on a promotional discount. If it’s way below what you’d expect to pay for it, it’s likely either stolen or fake.
5. Used items: they’re tricky. I get a bit squicked out by the thought of used clothes or shoes (so you’ll probably never see a post on thrifting from me) but I have bought plenty of gently used purses and jewelry items from eBay. Make sure you see photos from every angle (front, back, side, bottom, inside, zoomed in, etc) and double-check the seller’s ratings. Never be afraid to ask for more photos. Always trust your gut with a seller.
6. An exception to the beauty product rule: if there’s a Gift With Purchase going on at Macy’s, but I don’t want any of the Purchase products required to get the Gift, I’ll check eBay for the GWP. Usually somebody is selling it. Make sure you get it new and unused. I got a fabulous gym bag sized face palette from Bobbi Brown this way. I’d otherwise have had to spend $150 at Saks to get it.

Some of my biggest eBay success stories: Off-season colors of classic J. Crew work dresses. The aforementioned Bobbi Brown palette. TIGHTS. So many tights. (All NWT. Obviously. Ew.) Theory blazers. And, of course, gently used purses.