Gosh what a "tricky" buyer you seem to have struck.

Firstly in relation to warranties just to prevent another board furphy from developing - there really can be no blanket statement that warranties are not transferable.  Some are, some are not. 

For example, warranties for Samsung products  are usually transferrable if Samsung is notified of transfer of ownership. Many other manufacturers offer warranty transfers as well.

Generally from what I can gather via google Nikon does not provide transferrable warranties, however that does not negate a buyer negotiating additional terms with a seller for return of goods to them during the warranty period if a defect arises, for the seller to pursue through the original warranty.