If you don’t want to lose your shirt on eBay, I suggest learning about proxy bidding. The very most important thing, though, is to check your seller’s feedback before you submit ANY bid. A couple of negative feedbacks in a few hundred positive ones is no big deal, generally. I, personally, will never buy from someone who has no feedback (that is, this is their first eBay transaction). Or at least, I haven’t done so yet. Anyway, examine the seller’s feedback even more carefully than you examine the item. Are there a lot of negatives? Do the negatives say something like “Seller claimed it was lost in the mail” but the seller will not offer insurance? Look at the seller’s replies to feedback as well. You can pretty much tell who will be trouble before making a bid.
Examine everything on the seller’s page. Notice what forms of payment are to be used. Again, personally I won’t buy from people who specify “cash only”, and I very rarely use PayPal. PayPal is a big ripoff, IMO. However, a lot of people love it. DON’T bid on something that is “PayPal only” and then ask if you can write a personal check. Use the “ask the seller a question” option and ask about payment options FIRST. I’ve had several sellers who were quite willing to take a money order, even though the ad specified Paypal only. Look at the item description. If it doesn’t SAY “from a smokefree home” then it might very well arrive reeking of cigarette smoke. This has happened to me a couple of times. Once it happened with a glass owl-shaped bank, and I never thought of glass as something that would hold smoke odor, but it did. Similarly, if it doesn’t say that the home is petfree, you might have pet dander on the items, though I am considerably less sensitive to that, so I’ve never noticed it. The best sellers will describe any flaws in the item, and the very best will also include close up photos of those flaws, so that you can judge for yourself whether or not you can live with the imperfections.
Photos…I rarely buy an item unless it has a good photo. I acknowledge that this puts a burden on the casual sellers who don’t have access to a good camera and/or scanner, but I have a right to know what I’m buying. Even if it’s a book, I like to see what sort of condition it’s in. I love those sellers who will put up several photos of the item, each showing a different angle. One of the things that I like are older fountain pens, and so many sellers seem to think it’s OK to put up a photo of the pen with the cap on. I want to see the nib, at the very least. A nib close up would be better.
I have several favorite searches saved. They are quite handy. I’ve got a couple of favorite sellers, too.
Remember, there’s almost no unique items offered on eBay. If you miss one item, one just like it or at least very similar to it will be offered again, and quite possibly at a better price. So don’t be afraid to set a maximum price for an item and sticking to it, unless you’re bidding on something that is very popular right now. If something is EXTREMELY popular/collectible right now, I review my need for it. I can probably wait for a couple of months or even years, and pick it up for a quarter of what other people paid for it. As you can probably guess, I never bought any Beanie Babies. 
There’s a page called something like “My eBay Bidding/Watching”, and I use it. For one thing, it will keep track of the total amount of bids you have out. If you have bids on five items, and the max bid on each is $100, then you probably will not want to bid on another item.
Incidentally, be wary of purchasing movies, music, or software from Asian countries, as they tend to take a very lax attitude about copyright laws. You might end up with a pirated edition which is unplayable. For that matter, be careful about buying movies outside your region, as you probably won’t be able to play them.
