I raised about £2,500 a few years ago for a house deposit, by buying & selling bikes. Sometimes I made only £200 per bike, a couple of times I cleared over £700 per bike.
To do this, you obviously need some starting capital, tools to bring the bike up to a better finish etc and some knowledge of the bikes in general & those models in particular that you intend buying. I generally stuck to Suzuki & Yamaha (GSX, GSX-R, FZR, FJ & XT).
Because I was dealing with the cheaper end of the market, there's always the risk of buying a piece of shit that dies just having bought it. My rule was this: assuming the engine dies, never pay more for the bike than the salvage value of the remaining parts i.e.forks, frame, wheels & electrics etc. If it all went wrong. I planned to at least recover my initial outlay.
A good understanding of what sells and the local second hand spares market, should you need any, is also desirable.
Fortunately the above scenario never happened, I covered a few miles on some of the bikes and learnt a lot about models I'd never previously owned. One last benefit was that some bikes came with tools, spares and workshop items that I kept or sold separately (paddock stands, manuals, race cans etc).
Best of all, I had a good time doing it.