Mentioning Seller's Original Purchase Price.. Bad Form?

[Joe Ferguson]

Hey BP,

I am negotiating on a 4-plex where the seller is asking 319K and I need to get it to at least 280-290K to start making sense. 

I was amazed to find out that he paid 64K for it in 2013. Factoring in 6 years worth of cash flow, this guy is making an easy 500-600% on his 64K. I am aware that this is irrelevant but makes me feel better about being more aggressive. 

Is it considered offensive to mention anything regarding a seller's purchase price or profits in a negotiation?

Thanks in advance.