I have the same problem. I have a very niche app that I made for myself, then thought I might as well try selling it since there is no alternative to get the functionality.
I can't say I've had a lot of success. Most posts on Reddit get removed as ads and relevant Discords tolerate self-promotion even less. Got one upvote from HN [1] though. Perhaps if I'd written it in Rust instead of C, it might have done better here :)