To be fair, as far as I can tell a game developer has no way to verify whether or not someone has bought it on itch. As sucky as it is, it's itch's fault not the game's developers.
I think there is a way, because i had bought another game on itch and the dev had to remove, but he linked the account info with his patreon, i dunno about the shinnenigans he did, but it worked pretty well, i just cant remember what game it was since it happens like last year, but w/e, this is probs a way to make people buy the game again (if you want then go for it), but.... yeah... im not gonna buy again
(I aways find these paid games for free on a page that im not gonna say the name, and im not on any risk since its a dedicated community)