I Think as Tom says , need the customer to have the car for some time to make sure they are fully happy before asking , on the spot I feel is too much pressure !
I have a customer feedback form they post back , plus asking them to review on www.carbuyingadvisor.co.uk , some do it on there some on facebook but the form prompts them ! Reviews sell cars ! Give me a shout if you need any further help Rory.