I'll second everything thats already been said!
in highschool i bought garage sale stuff and resold on ebay for a decent profit.
also the junk / parting out cars business seems decent
any car bing sold for 500$ will probably have more than 500$ worth of parts.
I also did handy-man type work one summer in college...way better than a real job and once you get going its not hard to work for cash. i was getting 10$ cash, no tax,....which was way better than the other min. wage jobs in that town (college towns are known for not paying much since theres such a large pool of people willing to work)
If you like dogs, you could watch peoples dogs while they go on vacation, at their house....or yours if you happen to have a kennel area
if you have any craft skill you could make stuff and resell it at farmers markets and the such....I go to these from time to time.... if you want the specific specific idea i have in mind i can share by PM....I dont want to give away my money making secrets to just anyone!!!.
or put your skills list on craigslist...
maybe someone needs a welder/lawnmower/kids-bike-assembler ??