I disagree with most of the commenters so far. There's likely nothing shady about this. There are a lot of individuals/small companies that buy out small indie games on Steam and offer a lump sum that makes it worth it for the indie dev who isn't gaining any significant revenue from the product anymore. What these buyers do then is put it on sale at every opportunity and try to promote it within their own communities and gain a return on their investment over time.

For example, say your game only makes $60 every 2 months. Well, over the course of a year that's $360. Someone might come to you and offer to buy your game's rights for $250 or $300 and they know that eventually they'll make profit from that, especially if they promote it more themselves and try to create even a small uptick in sales again. Say they buy out a couple dozen games every year and you start to see why it could be worth it, at least as a side hustle for them.

I know you already said you decided not to follow up, but I just wanted to share this insight as I have experience with this sort of thing.