Deleting a chat message is a two-step process:
Step 1 - Tap on the chat option and select the chat you would want to delete.
Step 2 - Clicking on the upper right corner drop-down gives you the option to ‘Delete Chat’. (as shown in image below)
Please note: If a specific message or attachment has been sent from a seller to buyer (vice-versa), the former cannot delete it from the recipient's account.