Selling online is easy, but different sites are better for different items. Craigslist is a great resource for larger items, like furniture, that you can't ship and need to sell locally. You have the option of requiring pick up only, free delivery, or delivery with an extra charge. Just be prepared to haggle — most people search Craiglist looking for a good deal.
A site like ebay.com is good for smaller, lighter items, like clothing or small toys, that can be easily shipped. And don't forget about all those books you have lying around! Half.com (a company of ebay) makes selling your used books easy. All you have to do is enter the ISBN or USC number, and you're on your way to making some money!
Get the kids involved by having them choose a few of their own items they'd like to try to sell. Help them write the descriptions and take pictures to post online. They'll get a kick out of seeing their own stuff on the Internet, and will be equally as excited when someone wants to buy it!