I don’t use a service to screen a buyer (other than the park) and have had good luck overall. For me I want a criminal report, a credit report and references including that last couple of landlords.
Now as far as the criminal report make them get it. This is a bit of a test becuase if they can’t do this then they will not work out as your buyer. On the criminal report you want to just make sure they are pretty clean but they don’t have to be squeeky clean. For me I work in parks that will not allow anyone with a felony conviction so that is my rule as well (with one exception so far that actually worked out great… but thats another story). I would also not take a buyer who has any history of domestic issues as that is a big red flag.
As far as the credit report just expect it to be bad. The majority of lonnie deal buyers are in their current situation because they cannot take care of their credit. What you want to look for is someone who does not have a history of running debt up and running away (ie bankruptcies). But the caveat to that is medical bills. I love buyers who have only medical bill issues on their credit report because face it we are all going to do whatever it takes to take care of ourselves and family medically even if we cannot afford it.
Now references are also very important as they can let you get a feel about the buyer as far as how they pay and who they are. I learned the hard way to always call previous landlords and get their input on the person.
Overall these things though are just tools to getting the right person as a buyer. Personally I go with my gut more than anythiing with guidance from these tools. I also completely agree with Lonnies advice that you want to get someone who is happy to be getting into your home. If it is a move up for them then they will do what it takes to keep the situation going.
Just my thoughts,