Thing is most route games sell @ 60% or less than the "new" value. So I'd assume that he has an inkling on what second hand pinball prices are if he want's 20k. Brand new coin op pool tables can cost 4k or more, good luck getting anywhere close to that on the second hand.
If you don't want the games offer 15k, you might get lucky. if you actually want the games, offer closer to what he is asking. 18k is the lowest I'd offer if I actually wanted the games. (that's if they are as descent as you suggest)

I wasn't talking about pinball, other route games. Just making a point that the potential seller is aware of their value if he is asking 20k. Because other route games like cranes and newer arcades loose a lot more value than pinball.