Since you weren't excepting PayPal, wouldn't you be requiring either a check or money order? And wouldn't you HAVE that check or money order in hand before shipping out the book?
Exactly. For Randy the 3rd party option was in an important sense reducing his risk, and for the sake of concluding the transaction he even waived that idea, which was never therefore a condition, and offered to send direct payment.
Randy had proposed Rick as a possible 3rd party mediator. Frankly, if the seller could have confidence in Rick as a customer he could have trusted him as a facilitator of the deal with Randy, no?
+1