Here's a thread on Steam talking generally about ripped content and using assettocars.net as an example.
http://steamcommunity.com/app/244210/discussions/0/627456486611638358/?insideModal=1
Edit: To be clear, using ripped mods is a personal preference and if you want to use them feel free. It's frowned upon by the developers, racedepartment, and the community in general which is why some of us are quick to point out ripped cars (also, the few I've tried all handled/drove terribly) but that doesn't mean we're telling you not to use them. I would, however, strongly advise against paying for ripped mods because 1. it's blatantly illegal B. since ripped cars generally don't feel nearly as good as proper AC/scratch-built cars you likely will regret parting with that cash.