If the prior owner did not provide you with good information about the number of these gift certificates and properly adjust the selling price of the business accordingly, then you may have a good case against the seller.
It may be possible for you to refuse to honor those certificates, but you need to discuss all of the facts of the situation. Normally such certificates must be honored, but there are exceptions.
This is complicated and I can only advise you fully after discussing all of the facts.
Contact my office for a consultation on these issues.
Robert J. Spitz
909 395 0909