I'd almost have to agree with John at quick glance, very difficult to say one way or another looking at pics. I've seen a lot of first generation alterna x thayeri hybrids, a very large herpeticulture operation near me crosses them and all 50/50 hybrids that I have seen have the distinct alterna gray ground color. Your animal has a very classic dirty or speckled buckskin/yellow ground color. If there is alterna in there it is minimal, but possible... it happens. There are hints of something besides thayeri going on there but impossible to put a finger on. The one thing that really makes me wonder is the dirt cheap price, that signals the seller was desperate for some reason to sell the snake. But then again, the seller just may be real nice and gave you a bonus. I would politely ask them about it if you really are concerned and see what response you get. lol... the "het for leonis" thing kinda bugs me too.
The only way to be sure you are putting your money into pure animals is to pay the price and buy from known reputable breeders that can, and will gladly attest to the validity of their breeder animals.
But then, I am just a crusty 'ol stick-in-the-mud purist.
Good lookin' snake though...
Mike