What do you mean by “assure”? Are you sure the seller will have no tax liability or you simply want to make him/her believe it? If the former, then explain your reasoning to the seller, show supporting documentation, having CPA involved would REALLY help. If the latter, than it would be misrepresentation at the very least.
Oh, yes, by the way, the seller cannot be liable to the IRS for the difference, s/he can be possibly liable for the taxes on the difference. But this wasn’t what you were asking, was it?