I mean, you can't really fault new owners of a business if they aren't honoring existing gift certificates that they weren't given money for. Would it be nice if they did? Sure. But that could be thousands of dollars of revenue that went into the previous owner's pocket that you are asking the new owners to cover.
In addition, there may be tax implications for the business since often (always?) revenue for a gift card is not recorded until the gift card is spent. So if you honor a gift card a previous owner sold, then you are creating taxable income for your business when in reality you received none of that income (in addition to paying for whatever inventory is purchased with the gift card)
dor.wa.gov
EDIT: To be VERY clear, I am not an accountant. So please verify with a tax professional if it is relevant to your own business.