Like you said, is a question of habit. Is not to be introduce from the day to the night...
This is not like the question of the price, that you send a warning when is missing. Is not necessary to monitor if the PM was announced.
Like a ticket in a public service, you don't expect someone pass in front saying that a call was made the day before... also you don´t wait for your time without a ticket... and also the Service don´t care if you have not a ticket or if a guy in the Service is your friend.
The idea is here... could end with the PM never received, the problem with the inbox, the trust on the seller, etc... and write a phrase on the topic is even quicker than the PM itself.
The advantage is evident, the sellers are also buyers, and just who never had a situation with items already reserved could ignore the subject.
Thanks for your time.