my practice with a zero feedback buyer and seller is to proceed with appropriate caution, and base my decision to complete the transaction on the dialog with the person (ie email or voice). Depending on the transaction size, I may respectfully ask the person to provide personal backround to validate their credentials (and I usually apologize with an explanation that the web is a dangerous business-place, and offer to provide them more of my credentials if they like).
If the person isn't forthcoming when I make this request then the deal will not proceed. I've never had anyone balk at this request, by the way.