What's to stop VPN services from using/selling your data?

I' m fairly novice in my understanding of VPNs - but I understand at least one of the reasons that people use VPNs is to prevent ISPs from using or selling their data. So I can understand why people would go to a VPN service to avoid this, but what is stopping the VPN service themselves from doing the same thing and making a profit.

Also, I can see someone writing in the comments that only free or untrustworthy VPN services would do this. But how would a VPN service (free or otherwise) even be held accountable for that, other than potentially consumers finding out and there being some big public scandal.