The "I couldn't care less if you bought it attitude" seems to work for me most of the time and to be honest, I really couldn't care less if he would have bought it or not
I had very similar experience like this not long ago.
Enduser contacted me asking if it's available for sale.
I simply replied with it might be, but, I have plan for it.
He came back with $150 offer and said that would be fair since
I didn't do anything for many months and it's just parked.
I replied with brief explanation of what domain means to me
and what I gathered (contents, resources) so far to develop that domain
and I have personal attachment to this particular domain.
I ended up selling it for mid $x,xxx and he wired the money
to my bank acct in 3 days from initial contact email.
I never mentioned how much I wanted for the domain.
He did most of the talking, negotiating and selling.
All I did was made sure to let him know that "I hope you
do NOT offer top $ that I can NOT refuse." and I rather
keep my domain than money :D