Well, there is going to be two answers you can get, either he will say "no", or will counter offer.  Anyone who really wants to sell, will bargin.  If he has just started advertising it, he may wait to see if anyone else looks at it and offers him full price.

I have had my trailer up for sale off and on because I don't really need to sell it.  I still have two horses and will use it, but it's a 4 horse slant and that is more than I need.  So I can sit on my price if I chose, because I really don't want to be without a trailer, but really should just sell it and downsize.

So worse case scenario, if you really want it, and he won't budge on the price, then pay it.  If not, walk away leaving him your number saying to please call you if he can't sell it for more than your offer.