Sounds like a good deal to me, the Vulcan 500’s are nice rides and it already has the most useful accessories on it. It wouldn’t hurt to haggle over it some IMO, most people who sell bikes are always going to list it for more than they are willing to sell it for. I’d throw him a lowball of $2000 or so and hopefully end up meeting him in the middle somewhere. If your in an area were the riding season is just about over, you may end up getting a really good deal, as he may want to unload it to avoid storing it for the winter. As WeaponZero said make sure you find out the maintenance history of the bike and adjust your offer accordingly.
Good luck with the purchase. BTW, got any pics of your GoldWing Trike?