What you alluding to is called a "race condition." It has been previously addressed here:
http://www.tipsandtricks-hq.com/forum/topic/available-copies-and-multiple-pending-client-sales-race-condition
The reasons for a pending payment are usually because something "was funny" with the buyer's payment method. Yes, it is a form of economic bias; but "that's life."
Class seats are a form of physical product; I addressed that in the referenced post.