Ah, I wondered why the chain was hanging like that and the lamp was positioned to obscure things! Providing the rear wheel is original, it will require a six speed freewheel, as opposed to a cassette. The original gear range was 14-28T. You can buy Shimano, Hyperglide compatible freewheels in this range and number and cogs quite inexpensively. However, you should also buy a new chain, as the old chain likely will not mesh with the freewheel due to wear. Besides, the old chain also looks severely kinked. You're probably best to toss it. If you're doing the work yourself, you'll also need a chain tool, if you don't already have one Your LBS should be able to fix you up with all three.
There's almost certainly a reason that the bicycle only sold for $15. Without knowing what additional works needs to be done, it's impossible to say if you can turn a profit. Depending on whether you're doing the work yourself and the tools you have, you're already looking at $25- $50 to deal with the chain and freewheel.