I use E-bay quite often, and have had very good luck both buying and selling. It can be quite addicitive! I've used it to buy a variety of things. Mostly audiovisual equipment, and car parts. I've sold all sorts of odds and ends, like a phone system out of my church, a 3-tube video projector, an old refrigerator, etc. E-bay works best for smaller items that are easily mailed. I have heard of a few people being dissapointed about the condition of items ordered on it, where they weren't in quite as good of a condition as described. E-bay has a user feedback system that keeps people honest and minimizes this issue
They easiest way to use E-bay is to set up a PayPal account. PayPal is sort of an online bank that you can use to pay E-bay sellers with, and where your sales are deposited. PayPal is very secure...just watch out for Phishing (fake E-mails designed to look like they legitimately came from PayPal, and are designed to con you into entering personal info into them!) PayPal is connected to either a bank account, or a credit/debit card account, so if you pay someone, or someone pays you, you can then transfer money from your bank account.