I think, if you were to build planes with the expectation of selling them later, you'd want to target planes that are NOT available in ARF form. Or at least not in ARF form in the size you're thinking of building. It would be a difficult balance. You want to build planes that are popular enough that people recognize and hence want to buy them, but not popular enough that other manufacturers are already producing ARFs of.

I think if you go in with the idea "I'm going to build this plane, fly it a little, then sell it." Knowing you probably won't make any money at it. (Or very little) you'd do OK. If while you're building, you spread the word on what you're building, then by the time it's done, there already may be a buyer. Or take requests, so you can compile a list of planes to build.

I thought of doing the same thing, but I know I'd never get rich, or even make a living, doing it.