1) Feedback Matters
Feedback scores, though far from perfect, is still one of best and most expedient ways to determine whether a seller is trustworthy enough to do business with. And while any seller with a large score and a feedback percentage better than 98% is usually trustworthy, some of the power sellers who meet eBay's strict criteria earn "Top-Rated" seller status, which is an even stronger indication that they'll do right by you in any given transaction. Especially when you're starting out, avoid buying items (especially large ticket ones) from sellers with less than 98% positive feedback or only a few feedback points to their name. Not all of those who fall below 98% or are just starting out are bad sellers or out to swindle you, but those who are will have lower feedback and - possibly - a fresh account to distance themselves from past bad transactions.
eBay's feedback system is a little different than it was back when I started. Sellers can no longer give buyers anything but positive feedback, and some users believe this creates a power disparity that helps to protect "problem" buyers who don't pay for their items or attempt to defraud the seller. But the current feedback policy also removes the chilling effect that retaliatory seller feedback could have on a buyer when an item fails to materialize, or shows up markedly different than how it was described at auction.
Sometimes a rare item you desperately want may only be offered by a questionable seller, or the price might be incredible. At those times, it may be useful to go deeper: look at the negative feedback that lowered the seller's score, or look at the few transactions (if any) they've already completed. If the negative seems to be an isolated incident, or item they're selling is in line with the things they've sold successfully in the past, determine whether the item is worth the risk. If it is, and you go for it, and the seller turns out rotten after all, then at least you'll still be covered by the next rule:
2) Know the eBay Buyer Protection Policy
If there's a golden rule to doing buying things on eBay, this is it. eBay's Buyer Protection Policy is one of the strongest safeguards in place to protect buyers from unscrupulous sellers, but in order to use it, a buyer should be fully versed in its provisions. It can be tedious and difficult to understand at times, but it's there for your benefit, and the better you understand it, the better it can be there to protect you when a transaction goes south.
One of the most critical provisions: eBay/PayPal only allows you to open a dispute within 45 days of your original payment for the item. If you wait until the 46th day to open a dispute, then all your rights under the Buyer Protection Policy evaporates into thin air. So even if your seller is begging you to give him just a few more days to resolve your issues, if the 45th day is looming near, make sure you open your dispute before then. If the seller comes through shortly thereafter, then you can close the dispute with no harm done; but if he's just trying to make your BPP rights expire, then you'll have headed him off at the pass, and are ready to take the next steps if he doesn't make things right.
3) Search for "Completed Items" to Get a Sense of the Market
In my early years on eBay, I would often find myself getting into ridiculous bidding wars with other buyers that ended up with either them or me paying way more for the item than any rational person should. The best safeguard against this is to do some research on how much the same item has sold for on eBay in the recent past. Taking the same search terms but filtering the search to show "Completed Items Only" will show you all of the recent completed listings that match your terms.
You can learn several important things from these search results: How much, on average, does the item go for? How many of this item have been sold recently (and, by extension, how likely will it be that other listings will offer this item in the future)? Has this seller sold the same kind of item before (and would he be likely to sell another one shortly after the current listing)? All of these factors can help you determine what a sensible high bid should be, and in that way avoid overpaying as a result of a frenzied bidding war with no clear price ceiling.