This has been a long-time issue in the AI community, both with models and repaints. Essentially, it means not representing it as your own if you did not do the work and not profiting off of it if it is not your work. Obviously, one may not sell someone else's work; but commonly also not uploads, or minor changes/"corrections" then uploads, without permission. The reason for the restrictions on uploads is because some communities gained a bad reputation for their practices. Re-use or changes solely for use on one's own sim -- without distribution -- is considered fine.
So, all-in-all, anyone who respects other's work won't run into a problem.
My guess would be that, unless there is a specific statement that the livery is public domain, freely-usable, etc., then a claim of copyright probably could be made (as with any other artistic work). Whether a case could be brought to court is another matter -- unless there is clear profit being made by large-scale re-sales. For the average simmer, it is more a matter of gaining a reputation as a thief.