This sounds like something that is not usually done in the indie sector. It is common in the "non-indie" sector. But usually it is the whole game studio that gets bought and sold.
Buying a single game from a still existing studio? Are there examples? Professional ones. I have seen hobbyists to try sell their source code for a couple bucks. And I would guess there are unfinished games that changed studios. Or more often, as was said, publishers.
There is nothing inherently ethical or unethical about all this. But in the low budget indie sector, there is the issue of trust and the problem of marketing. If you are a successful indie developer, selling out can cost you or the project fans. If you are an unsuccessful indie developer, why would anyone buy your unsuccessful project? The buyer would have to market it and maybe finish it. And here might be the point where you want to establish yourself, if I read your intentions correctly. Buying such unfinished gems, polishing them and marketing them. Which sounds like a hard thing to do, and offering "only" publishing services sounds a lot easier in comparison.
Also, in the indie sector you have to be especially careful about the legal baggage of things you aquire. Amateur single developers might not have been overly knowledgeable in the legality of the assets licenses they use. Stuff like using non-commerical items and such, for example.