I'm sure you'll get a lot of responses from guys who "flip" (buy, fix up, then sell) bikes.
Here's my thought (and I've learned this with cars and boats, but I'm sure it applies to bikes, as well): You'll never get out of it what you put in. Unless, like stapfam said, the bike was worth something to begin with.
Now - all that said, I'm teaching myself to work on bikes and am finding it to be a labor of love. If that's the case, then go for it...just don't make any plans with your future profits