How to Verify Norton Product (Specific Key) Not Already Used By Someone Else

I have an opportunity to buy a Norton product from an individual who is not personally known to me, although on the surface this person seems sufficiently trustworthy for an interaction of this magnitude.  For discussion purposes here, presume the product to be (what he claims to be) a new, sealed OEM box of 3-user Norton 360 V2 on CD.  If I was to buy this as a strictly private-party transaction, is there a way that I could quickly confirm (or deny) that the Product Activation Key was still valid (unused), short of installing the software on one of my computers and then seeing whether or not it would activate under my Norton account/userid ?  Also, if all three license instances were still available to me at the time of purchase, is there any way I could restrict anyone else from using that key in the future, since the seller could theoretically have written down the Key Code before sending it to me and might try to use it himself, or re-sell it to anyone else, later?  There is no evidence of fraud/theft, etc - the product seems to have been originally bought by him on the up-and-up; he claims to have never opened the box, examined its contents or used any of the three activations.  He simply doesn’t want/need the software and is trying to get rid of it for whatever he can get.