i avoid the customer test driving the car the very best i can, tell you this, they dont know the area, they dont know the car, they like to brake hard [ just testing brakes mate] they have no idea of the speed limits, my very best advice, over 25s only, even if your policy says over 21, always but always accompanied, be on your toes at all times, AND be aware of theft, if they do get to talk you into them driving, you drive first, you explain the controls, on swap over, you remove the keys, you hold open the drivers door so customer gets in, you then get in passenger side, and THEN you hand them the keys, one of the very best ways to avoid customer driving is not to go back to base on the same road, as in, go rouind in a circle, all left or right turns if you see what i mean, this way your back at base before customer realises, then you say "oh sorry, you wanted a drive didn't you ? i got a bit carried away talking to you, sorry" and i can guarantee 9 out of ten will say, thats okay, seems good anyway, as you've already demonstrated the car good to them, you've told them clutch is halfway, brakes are at top, you've demonstrated it doesn't "pull" either way. This way youve avoided any bumps, and youve avoided a stain on seat