Customary Inequities in Seller-Buyer Relations?

Why are appointments deadlines for the buyer but conveniences for the service provider? A buyer who misses a deadline, even by a few minutes, will have to reschedule regardless of any inconvenience to himself, whereas the seller can force the buyer to reschedule at his merest whim, even if the buyer was there on time.

Likewise, consider mail-order purchases. Sellers inform buyers that (1) the buyer must arrange for the seller to receive payment with a certain amount of time (usually a week) and that (2) the seller is not responsible for however long it may take the delivery service to get the purchased item to the buyer. Why doesn't EVERYBODY see that those expectations are unfair?

Aren't sellers and buyers coequal parties to a transaction? Each of them has something the other wants more than what he's offering, so they swap. The category "seller" and the category "buyer" merely indicates which party brings the cash to the trade. A distinction without a difference.