Before there was craigslist (like, 20 years ago), my dad used to pick up old wood furniture pieces (dressers, armoires, etc.) at garage sales or that were being tossed out, strip and sand them, and re-stain them to sell. He typically made about $200-300 profit per piece in 20-year-ago-dollars, and I'd estimate it took him about 4-5 hours per piece on average. I haven't done it myself, but I'd think that if you're in a good sized urban area you could make a decent amount of money doing this. Especially where there are high income people that are willing to spend big $ on furniture at places like Pottery Barn, Restoration Hardware, etc.
Heck, if you could turn around just 10 pieces of furniture a month at $300 each, that would be $3000/month for about 40-50 hours of work all month (or a 2-3 hour workday)!
It would cost hardly anything to give it a try. You don't even need any expensive tools, although a palm sander would save lots of time and certainly be worth the small investment. Sandpaper, stain, some rags and paint brushes, and you're ready to go! The trick of course is to try to pick up quality pieces as much as possible -- nice hardwoods, nothing warped, rotted, or overly falling apart that can't be fixed with a couple of screws or some new hardware. If you can recognize and score an occasional antique or rare piece there's probably significant money to be made.
I think if I was going to do this, I wouldn't necessarily turn around and flip it on craigslist. Instead, I'd probably try to operate more as a "professional" and high-end looking business with a website, some modest advertising, etc., and a vibe of selling custom, fancy stuff. That way you could draw high income people who wouldn't consider buying something second-hand on craigslist.
There, I just wrote your business plan ;-)
