I love eBay, and over the last 7 years I've had hundreds of great experiences, and just a handful of unpleasant ones...it's just like life, there are a lot of great people out there but from time to time, you run into a problem.
I have managed to avoid the ultimate bad experience of getting completely ripped off. eBay is generally very safe, but it's not immune from scammers...every day there are literally millions of good transactions, but a handful of cons where people lose hundreds or even a thousand bucks. You don't want to be a victim of fraud, so I'll give you just 3 simple pieces of advice -- follow these and you'll be fine.
Tip #1, buying safely is all about how you pay. The vast majority of sellers on eBay accept PayPal or credit cards. If you pay with your credit card, you're completely protected against fraud (no item, or item substantially different than described). If you pay with PayPal, look for the PayPal enhanced protection logo...about 3/4 of the items on eBay bear this mark, they're on items offered by sellers who have over 50 feedback that's 98% positive. Pay these sellers with PayPal, you're covered up to $2,000. And even better, those sellers never see your credit card number or bank account number, so you've got even better protection against credit card and identity theft.
What about the items on eBay that don't have the $2000 logo? They aren't necessarily unsafe...there are a lot of good sellers that only accept checks, money orders and the like. If the item price is cheap, I'll buy from those sellers, but I'll never spend a lot of money that way. Most items that don't have the enhanced protection still offer PayPal, and those items are covered up to $200, so your protection is more limited. Generally, you can find the same item from a seller that offers credit card or enhanced PayPal protection, so I'd stick to the safer methods, at least until you're more comfortable with the site.
Finally, on payments, never ever pay with cash. You get no receipt, no protection against theft in the mail or bad sellers that way. And worst of all, never send cash through Western Union or MoneyGram. Criminals all around the world use these services to trick users into sending them money, it allows them to pick up cash at thousands of locations around the world in virtual anonymity. eBay, Western Union and MoneyGram all tell you that you should never send money through those services to pay for an eBay item. Listen to them. And if some seller tells you that he needs the money sent in cash via Western Union or Moneygram because he's travelling on business or on vacation, RUN away from the deal.
Tip #2: Never trust emails. The Internet email infrastructure is very insecure, and it's clogged with spam, viruses, and "phishing" emails...that is, very convincing emails that LOOK exactly like emails from legitimate companies, but are in fact just phony emails designed to trick you into giving your personal finanical info or password to some crook. I don't care if the email says it's from eBay or PayPal or your ISP or your bank, the "From" line is easily forged. I don't care if it has the official color logo of the company, that's incredibly easy to copy. Fake emails are now very common, and it's virtually impossible to distinguish them from the real ones. So what do you do? If the email has some urgent "call to action" (we've lost your information, we're confirming your account, we're suspending your account, you've won a computer blah blah blah), it's probably fake. If the email wants you to clink on a link and go to some site to confirm some info or sign in or give your password, it's DEFINITELY a fake. Don't even click on the link. Delete the email. If in doubt about your account status with that company, open up a new browser, and go directly to that site yourself by typing the url in the browser...sign in. If there's a problem, the company will tell you...eBay will put the email in your "My Messages" which are on your my eBay page. But bottom line, don't trust emails...and don't click on links in emails. Ever.
On a related point, don't do business "off" of eBay. If you're bidding on an item, and you get an email from someone offering to sell you the same item for less, delete it. If you lose an auction, and get an email from someone telling you that the buyer backed out and the seller will now sell it to you, ignore it. In a few cases, buyers have reported getting fake emails purportedly from eBay assuring them that the item is in some bonded warehouse, owned by eBay, and directing them to pay by Western Union. eBay doesn't have any warehouses, and (remember Tip #1), you're not sending cash by Western Union to anymone, and (remember Tip #2), emails are presumptively not to be trusted.
Tip #3: Look at the feedback. The feedback system is great, so use it. Don't buy from someone who has 0 feedback unless you're willing to risk the money, and look at the feedback percentage. 95% is NOT a good percentage...look for sellers at 98% or preferably higher. It's a good indicator of whether you'll have a good experience. Also, the detailed feedback is great. No one's perfect, but if someone has worse than 4.5 in a particular category like "item as described" or "shipping time", pay attention to it. Shipping cost scores are typically a bit lower.
At the same time, the feedback system isn't perfect, so don't treat the info in there so blindly that you forget Tips 1 and 2. Accounts can be stolen, allowing a bad guy to use someone else's feedback on rare occasions. And sometimes people are afraid to leave negatives because they don't want to get one in return, which makes some sellers look better than they really are. So really read the feedback...some positives really aren't that positive once you read the comments. Look at the negatives and neutrals...an occasional one is ok, since no one is perfect and sometimes buyers are unreasonable. But if a seller has a lot of negatives, it can mean trouble...read the negatives, and read the comments that the seller is leaving for other people, it can save you from running into a bad merchant.
Now, if all the above sounds a little scary, that's ok...that means you're well armed, and better educated than most of the other buyers on the site. And the odds are, you won't need any of this advice, and you'll go for years and many transactions without a problem...but if you run into one of the Internet con artists that pops up once in a while on eBay, you'll be ready.