My question is are these companies legal or are they scams
Both. Beware that these prices are subject to inspection. When you take the car to their depot, there's more than a good chance that they will alter their 'bid' (downwards) once they have carried out their inspection.
If, on the other hand, you are happy with their final valuation, make sure you have cleared funds in your account before handing over the keys and signing any registration papers.
JJ
You gotta tie yourself to the mast my friend, and the storm will end.