As they have been modified, I would look deeply at the rest of the drums, play them with a guitarist, if you can get one to go with you to look at them, then make an offer you are comfortable with, in the 500-700 range. Modifications kill value, and the sound is only up to your ears. If he doesn't like your offer, simply walk away.
Let's look at a Ludwig kit that has been altered...what would have been a $1300 kit, then modified, say taking the rail mount off and replacing it with a 70s Ludwig post, will half the real value, although it will not make much difference in the sound. Unless there are a lot of really nice cymbals included, I'd make a low-ball offer and leave it at that. Afterall, the only thing "special" about these drums is the material they are made of, either fiberglass or carbon fiber. Nothing else matters now that they have been jacked with...and take only as much as you plan on paying, not the total amount. That way, you are the only warm body he has had in front of him with actual cash in hand. Trust me, he will take cash on the barrelhead...many times these kinds of ads draw a ton of email requests for all sorts of information, including low-ball, sight unseen offers. I promise you, he as turned down $500 in email lowball offers, but once a real human shows up with $500 in cold, hard cash in hand, he is likely to take it. If I were going after this kit, I would take not a penny more than $700, then offer 500 and lay the cash on top of the floor tom. Make sure you take the cash in $20 bills and make sure you keep the first $500 separate from the rest of the cash. You don't want him to know, for a fact, that you have more cash in your pockets. Put the first offer in your left front pocket, the next 40-50 in your right front and so on. Keep it as well separated as you can, making him think you only have brought so much.