I've found it to be a good way to purchase and sell items both. Selling items, you can eliminate the hassle and risks of having someone local come to your house to buy the product from you, giving them access and knowledge of your household, and it's belongings ect... Using classifieds for example, I could easily come to your home, see all that you have, ask you a few questions at random, and know when you will be home and not, as a result. So good bye to the rest of your stuff lol. So Ebay is great for that. People have to pay before you ship, so you are generally protected in those regards.
Buying things from Ebay, that's typically a little bit less cut and dry, but if you are smart about it, and how you identify trusted sellers, it's a great thing. Be watchful of the reputation feature, and feedback on items being sold. That will keep you from being ripped off. Also be sure to read each sellers terms regarding returns and refunds ect.. When dealing with some of the major companies, say for example "Verizon, Sprint, At&t", you can find some great deals where Ebay has gotten a contract to sell items for them, and offer a discount you'd not normally find anywhere else.
My general rule in life, though greatly criticized at times, is "Trust no One!". Does this mean I'm locked up inside a bubble? Not so much, but don't think I'm not looking you up and down on first meet, looking to see if you're going to try and bust a move lol. In other words, keep your eyes open for crooks, and shop safe!