I have talked to a couple of dealer sales people in my area who told me they have done deals where they have sold bikes with a "subject to" test ride. Basically they write up the deal, then you test ride the bike with the clause that if you hate it, you can back out of the sale.
I always take advantage of the dealer demo rides, so let's just say I have rode my next purchase a few times!
Personally I have allowed some test rides on bikes I have sold, but the majority of the time I tell potential buyers that I have no insurance on the bike, & can't allow them to take it on the road.