Here's a very clear example:

Eternity has all the information above their art. If you can use it profitably or if you are NOT allowed to sell it but can use it freely. They also have strict rules on offsite posting.

Say I have a free to use base of, lets say a dragon. If I credit it properly, but add my own colors/backround to it, since it IS a f2u, would I be able to sell it as an adoptable or something along the lines of that?

im pretty sure you can, as long as the creator of the lines doesn't say "non-profitable" or things along the lines of that

Yes, you can. Before using F2U lines check for offsite posting, if you can make a profit off of it (if you're selling adopts) and if it's still available for use. If it says P2U(Pay to use) you must pay an amount of currency before getting ahold of the lines. You are not allowed to sell/make profit off a blank lineart that is not yours though. I hope that makes sense.