This is exactly what I do. Find an under-priced bike, to some quick TLC to it (Clean, inspect tires and brakes), and tack on a markup.

I generally look for bikes that will cost me less than $30 to get road-worthy, and then tack $50-$75 on the price. And, I re-post it in about 30 minutes after I get it home (Doesn't take long to clean, inspect, and replace tires/tubes).

I don't see anything dishonest with this. This is what keeps bikes out of the landfill. Even those BSO's.