An update. A pretty late one, mind you, but nonetheless... "he" is, in fact, a SHE. Seller refused to comment when I made that revelation. And she still can't fly, even with her tail in. Curious. Now, I've never seen anyone talk about clipping a finch's wings, so I've never seen what that looks like. I didn't think anyone did that that often, but, turns out, this guy does and never told me. I can now see that her wings are definitely clipped, which explains why she still can't fly. Seller yet again refused to comment on that, just acted like I didn't even ask the question.
So, uh, yeah. Figured an update might have a slight chance of helping anyone in a similar situation. I didn't really think people clipped finches wings like this. I guess I never even thought about it. So, unfortunately, at the end of all this, it turns out the guy is just super shady. Incorrectly sexed the bird and clipped her wings without telling me. Guess she'll just be a hopper for a while. Quite depressing to see, though.