proceed with extreme caution....If your bike can carry a pillion, take the buyer on the back for a few miles, then swop over and let him ride it back. (Making sure there is insurance cover for this exercise FIRST)
or....if you have a second bike, tag along with him/her (which I've done a few times)
Make sure they know they must bring a valid driving licence (both parts so's you can check the address) AND their insurance policy showing SAME address AND cover to ride a bike not owned by them (even if you have your own cover)
If they come with a mate on a bike or in a car, ask for the vehicle keys for the duration of the test (even if you go with him/her)
good luck