[quote name='jimflox']Required, no. Nobody is "required" to say "Thank you" "excuse me" "you're welcome" "please" etc either. It's just common courtesy/decency.[/quote]
Feedback is not for the customer. Feedback is for future sellers, and that feedback has to be honest. The feedback can not be honest if the seller can not leave the deserved negatives for deadbeat (or dishonest) customers.
The THANK YOU note is for the customer,
and if they receive that courtesy (plus the item they bought) they have no reason to complain.