The balance of gift certificates outstanding should be listed as an advance deposit on the balance sheet and treated as a liability until they clear - assuming the current owner recorded them properly. Then, when they clear, the revenue is recognized. Accordingly, as a liability, it would reduce the value of the business as part of owner's equity at the time of valuation and subsequent sale.

In the case where the seller does not know the value, hopefully there were records of money collected in the deposits and noted as such. That could be determined through an audit, assuming there was enough to be concerned with and a journal entry would need to be made as a liability for that amount on the balance sheet.