Just tell the guy.....The boat is what I've been looking for but I just can't spend $xxx on one. Hope you find someone willing to pay $xxx for it but I'm going to be in $zzz range. If you decide to come down quite a bit please let me know. Be prepared to walk away...whoever calls the other first, LOSES!
I used to buy and re-sell express cruisers, this was my approach. I got some and lost others but if someone is willing to consider the range your in they'll speak up.