As the seller, unless the buyer specifically asks for it, you don't need a service. Simply update the nameservers to point to the buyer's server and collect the payments on a monthly basis. If they don't pay, or they use the domain for something you feel could hurt the value of it, switch the nameservers. When they've completed all the payments, transfer it over to their possession.
I personally purchased a domain on an installment plan. I gave the seller a 1/3 downpayment and told him I'd pay the balance off within 90 days. If I did not complete the balance within 90 days, there would be a penalty fee. He agreed and I paid before the 90 days. You can do something similar.