On Going Commission... Forever?

I’m trying to figure out a way to pay my sales rep an on going commission on the sales he makes without having to keep paying him if he stops making sales. Here’s the deal.

He’s going to sell listings to auto dealers on my auto site for $10 a listing. I want to pay him $5 for each vehicle listed. So if he sells 10 auto dealers who each list 60 cars a month (600 vehicles) he makes $3000 ongoing as long as the dealers keep listing. But if he quits selling for me, how do I NOT keep paying him for the next 7 years or whatever. There’s only so many dealers locally and say he’s sold them all, now he has nothing to do but keep getting paid. I’m trying to figure out something that’s fair for him and me.

Any suggestions?

Thanks You,
Jason
http://SpokaneCars.org