That's disgusting.
First of all, I doubt the guy that sold them to you will take them back -- it has been 3 weeks and I don't know of anyone (aside from Serpent City because he is a vet) that has any kind of guarantee on live animals longer than 24 hours to a week, MAX. They have been in YOUR care, so unless you can somehow prove that the regurging is caused by a problem that originates from the seller, he has no reason to give your money back or take the snakes and risk infecting the rest of his collection. The only way you can prove they have anything that came from the seller is to go to a qualified reptile veterinarian -- which you should be doing ANYWAY.
These are living animals, not toys, not disposable objects or trading cards. They need health care and they are YOUR responsibility so you need to provide it. If you don't want to pay vet bills, then don't own any kind of animal whatsoever. Because that is the only way you can get around it.
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~* Jen *~
Pink Lady Constrictors