I sold a car recently, which had finance. I decided up front to be completely open about it with the buyer - and asked them to pay the finance co. directly. I provided them with a dated letter from the finance co. with a settlement figure on it, instructions for payment and so forth.
I also informed them not to pay me the balance till they'd paid the finance company - and to advise me when the company had been paid. This of course was all written down in a contract.
The seller seemed perfectly happy to do this, and also requested that the finance company advise both parties by fax when the amount owed was settled.
All paid via Chaps (I covered the costs since it wasn't the sellers cost that I had finance on the car), all sorted reasonably quickly.
I also waited for the finance company to tell me that the funds had cleared - effectively I couldn't release the car to the buyer until the actual legal owner (the finance co.) told me I could. Essentially the finance house acted like an escrow account.
Easy to do - just make sure all parties are informed and all sign a contract.
J.