Looking for 18+ manhwa where MC finds ex-gf in a brothel after a lot of years. Finds out that she used to be sold for sex by her step dad. Cant remember much more

"18+" and "manhwa", I'm guessing we're talking about a webtoon......or not?

Well, I'm just guessing it is........mainly because I'm not sure I've ever seen a borderline H traditional (print) Korean manhwa before.........well, I'm not a big fan of the borderline H Korean works, but if that's what you're looking for (and it was a webtoon) I'd bet money it was a Lezhin work.......

https://www.lezhin.com/en

Again, if it was a webtoon, take a look around their site and I'd think there's a high percentage chance it'll be there.

Good luck 🙂

edit: Here, come to find out there are a small handful of traditional Korean manhwa tagged as Borderline H...

https://www.mangaupdates.com/series.html?category=Borderline%2BH_-Webtoon_-Web%2BComic&type=manhwa&perpage=50

or at least those titles don't carry either the webtoon or web comic tag as well........you might take a peek at them if it's not a webtoon we're talking about.