This is one of the reasons I don't sell my vehicles privately when it's time to say goodbye to them. I don't want potential thieves coming by casing out the joint, I don't want to deal with the test ride question, I don't want to deal with the bogus cashier checks and other financial scams so common these days, and in general I don't want to deal with all the uninformed idiots and looky-loos that seem to be unable to resist responding to sale by owner ads and wasting the sellers time.
While I wouldn't consider letting someone take my bike for a test ride, I can definitely understand why people would refuse to buy without one. I wouldn't consider buying a used bike or car or bicycle without a test drive/ride. For that matter, I wouldn't buy any kind of used product without testing it first. Once you hand over the money to some private individual, good luck getting your money back when you find the product doesn't work properly or has many undisclosed defects. These days all too many private sellers are no better than the old stereotype used car salesmen (or the modern version, Wall Street bankers).
Jerry