You might feel ripped-off because the thing you want to get rid of was a birthday gift, and it would feel really good to make some cash, wouldn't it? First, though, you have to take a realistic look at what you have. Yes, there's currently a wave of nostalgia for old toys, for example, but yours probably isn't worth selling if it's not in good condition. You could sell a piece of furniture that's well-made and in need of refinishing but probably not a cheap one that's falling apart. If you have a gut feeling that you have an antique or something truly rare, get it appraised before you ask way too much or way too little. You have a lot of options for selling, so I'll just look at the two powerhouses.
Lots of people sell things through eBay, and it's very user friendly. If you have no clue what your item is worth, you can view completed listings to see the going rate. Plenty of potential buyers are waiting by their computers, and you can get paid fast. Selling on a site like eBay has drawbacks, though. Long-time sellers have the advantage because they know the best ways to work the site, and people feel more comfortable buying from someone who's been around longer. Furthermore, eBay charges you for the listing and takes a cut of your sales. You have to decide things like what type of payment to accept and whether you're willing to ship overseas. Then you have to take the item to the post office. Many people prefer eBay for small items like clothes and toys -- shipping can get expensive fast.
Craigslist has become a popular way to sell stuff, but it's totally different from eBay. It's more like the classified ads in a newspaper (which people still use, believe it or not). You might have a hard time figuring out how to price an item but allow for some haggling room. It's free, and it's for locals -- meaning the buyer will probably want to come to your house to pick it up. It's a good option for large items, like that treadmill mocking you from the corner of your bedroom. Craigslist isn't as easy for buyers to search as eBay, and since it's free and doesn't require any sort of registration, you're kind of on your own. That means it's easier to get cheated. Still, if somebody's willing to pay you and pick up something that you'd otherwise have to figure out how to haul away yourself, it could be worth it.
If you're lacking time and motivation or you know that your stuff isn't worth the trouble, you still have options for making it disappear from your life. Some of them might even make you feel good about yourself.