I recently sold a vehicle and a truck cap on Craig's list. One thing I've learned is that many Craig's list buyers will low ball right out of the chute. It would not surprise me in the slightest if people have looked at this or called and offered $3500. That being the case, many sellers set their price way high so that the low ballers will be less inclined to look, or at least come a little closer to what they really want.
So what the seller is probably expecting is someone like you to come along and make a reasonable offer. If he really holds out, make a final offer and tell him to call you if he's interested in selling. Then walk away. Make sure you tell him what the NADA guide prices are. Keep in mind too that NADA lists the retail price, not private party value. Retail is what a dealer might get for it.