How well do you qualify buyers?

[Minna Reid]

When selling w/out a realtor? Do you ask them to prove they have financing? Anything else I should be qualifying for?
I wouldn't tie myself to a buyer with a ridiculous close date, or one with many contingencies, but how do I go about making sure that if I agree to sell to them, I won't be stiffed later with financing fall through or some other such detail? I can't afford to have the property off the market for a deal that would eventually fall through.