Hmmm. This sounds a little too good to be true. My gut feel is that they are looking to get a peek at your source code, and they'll want that before they give you any money. Keep in mind what PG says about deals, assume that it's not going to happen and just keep doing what you're doing.

That said, this is a great, low cost way to experiment with this type of interaction. If I were you, I'd set some strict goals as if it's a genuine offer (e.g. minimum sale price, not showing any source code pre-receiving money, etc), and then see how it plays out.

I've been in similar situations to yours before, and It's been really valuable to learn and make mistakes at small scale. As soon as it becomes a pain and your not getting much out of the experience, just finish it up on your own terms. The important part being that you should know what you want if it was real, and what you want out of the experience. Then either way you have nothing to lose.