Taking a duplex listing - negotiation

[Matthew Rubsamen]

Hello Bigger Pockets. I am a realtor and have several investors whom I am interested in showing a aprticular duplex to. 

I have met witht the owner of the duplex and proposed that I list her property however she wants more than the market will pay - that is a fact. Lets say for example the home is worth (i believe it to be) $500K - and she wants $540K, no if ands or buts about it.

She has also stated that the home gets a total of $3600 per month in rental revenue which I find hard to believe. What would you do in order to suggest the listing price of $500K and to see the paper trail of the alleged $3600 in rental income?