Seller only wants a cash buyer, but why?

[Joseph S.]

I've been eyeing a 3 family in the greater NYC area.  It's priced around 400k.  I made an offer and the seller liked it, but said he was holding out for a cash offer for capital gains reasons- he purchased it for under 100k years ago.

My question is what difference would be made in capital gains whether the property was bought with financing or with cash?  It seems the only way he could benefit would be to hide the actual sale price of the house when getting all cash.  Even so, that seems like a bit of a stretch.  Or maybe not.  

The house in in great condition and would be a fine candidate for financing.  It would also likely appraise for the selling price. 

Any help would be appreciated.