I would talk them down. Asking price is rarely the final price. Remind him that its off season... if he's a motivated seller, he will take less.
I see about $200-$250 for the trailer and another $150-$200 for the boat... I would try to talk him down to the $350-$400 range.
That's just my opinion, and the market I'm use to in my location--they may be able to get $600 for that in your neck of the woods.
Good Luck!