This is an explanation of what I have observed when I have contacted a seller. This might not be exactly what goes on behind the scenes.
1. Click on button to Contact Seller, type message, click Submit
2. AAPK sends an email message to the email address that the Seller used as the contact email for their store
3. Seller receives email which, in addition to the message, contains your email address (i.e., the contacter's email address)
4. Seller responds (hopefully) to your email address
5. Additional exchanges between Seller and Contacter occur directly in email outside of AAPK boundaries.
I don't know of a way to confirm message was sent, or message was received.
I believe it is mandatory for Store owners to be members of AAPK. If a store owner is a member of AAPK you can go to the "About Us" link for the Store where you can see the store owner's AAPK User Name. From there you can opt to send a message via the PM capabilities of AAPK where you would be able to track Sent and Read just like you can for any PM.