while most of these 3d models are available for free, on 3dripper.com library or rigmodels.com,
is it legal to get these models for free and sell as NFT ?
You have fallen into the trap of thinking that an NFT is the thing itself.
An NFT is associated with something. It is not that thing. Owning an NFT doesn't mean that you own the thing it is associated with. You just own the NFT.
Here's an example:
This is the SHA-256 hash of your post: 49D06F5C298BC68E14A652D88056041B8EFF795B5D156E63D8D782C62B0C49C3
The hash is an association with your post because only your post has that hash. I could use the hash, along with a URL, to make an NFT of your post, and I could sell the NFT. Now, would I be selling your post, or just the hash of it, or just some abstract association or representation of your post?
Now, to answer your question:
Laws vary by jurisdictions, but I don't know of anywhere that is illegal to sell an NFT of anything, unless the seller is committing fraud or is violating trademark or copyright laws in their effort to sell the NFT. IANAL