“Why not just stick to your job and play it safe?”

This isn’t the GFC, no doubt.

You’re just trying to make a few bucks to buy your own toys without committing to being a full-time shopkeeper.But here’s the thing: Wholesaling without a license is like running around the playground trying to sell toys out of your backpack.

Nobody becomes a millionaire in a few years, starting out with 10k, yet a lot of people try it. 

As others have said, either the paint was oil based and you used latex based paint over it but in any case, remove the old paint and use oil based primer (we swear by Red Kilz) to seal it and then paint over it with a latex flat ceiling paint . . . you should be fine.If the ceiling is drywall, make sure it is not wet indicating a leak from above (either the roof or from something above it)

“Why not just stick to your job and play it safe?”

Why should they change?

My take is to try out MTRs, and if you don't like it, then reconsider your decision.

Is the title of your post to be why AREN’T more people investing there or is it truly why are they?

Since he did minimal work and sounds like you are good with what you paid him (or should be good with what you paid him) that is really all that I would be worried about, and make sure he does not come back and try to put a lien on the place prior to selling stating he was hired to a job and was not paid.