Oops, yes you're right, it is a percentage of the net. I suspect that the contract in question is a relic from the sixties, when Hollywood accounting was nowhere as sophisticated as it is now and a percentage of the net actually got you some money.
Thanks, everyone, for the great advice. I'm going to ask the guy about the lien(s). The seller is actually a car dealership IRL. What's more, they actually stated themselves in the eBay auction what the KBB price is.