Yes, you're exactly right in thinking the machine is trying to pay out the credits. There is no simple way to "clear" the credits. The machine will either need to physically count them out of a coin hopper (which I'm assuming the machine is not equipped with), print a ticket (again, from the ticket printer it doesn't have), or have the attendant (you, in this case) "key" off the payout. Buybestslots is correct on the keys. Try it and see where you get.
(The seller should have informed you on how to cash out the machine, but maybe he truly didnt know how. I hope to God he was just an individual and not someone who sells machines regularly (I shudder to think it, but weve seen it before), as they should be embarrassed, well ashamed really, to knowingly sell a machine that isn't setup to cash out in one way or the other.)