You may actually be right about the Mojave, it does have a Mojave like pattern with the yellow/beige on top, and the greyish coloration on the sides. It also has some blushing, so it might be a pastave that is not showing much of the pastel.
Remember there are some animals that show their morphs well, and some that don't show much. The only way to be sure would be to breed him to normal females and see. Since both pastel and Mojave are dominant morph, you will see in the offspring what comes out.
I would definitely buy the animal if it was me, but would bargain the price down because of the poor condition of the sheds. Also maybe help the pet shop owner a bit, and tell him to give the snakes a bigger water bowl. Seems it's pretty dry in his shop.