As you describe the use case, it is not possible. As you suspect, there's no way to link one inbound message to a particular outbound message. I'd suggest borrowing an idea from Twitter and including a Base-36 code in the notification. If a reply contains that code, then it's in response to that original message. For example...
Notitification: A seller is interested in your widget. LFLR
Reply: @LFLR Sorry. We are sold out of the widgets.
It will take a slight bit of effort from the sellers. But, then, they are motivated to sell. Using a Base-36 code will keep the number of characters to type under five even for a million plus messages.