It was pretty disastrous. As soon as money was at play tons of people re-uploaded other's free mods and tried to sell them. They even tried copying their steam profiles to seem legit. There was another can of worms where paid mods would use assets from other games or made by other people. Aside from all the attempted theft, there was also tons of spam and fake/unconfirmed mods lying about what they are or trying to upload the same thing multiple times under different names to appear more in search... Etc...

Moderation didn't keep up and the whole thing collapsed on itself. Mods shouldn't be paid IMO, it just encourages terrible things rather than people making content for fun.