Most marketplaces email you when something sells. BookRouter™ can email you as well, per marketplace, at the moment the order is downloaded. For Alibris it is not a duplicate but a replacement, which is why it is on by default there.
Turning it on
Go to Routing › Marketplaces, open the marketplace, and find the Sync Rules card. Switch on Email new orders and save.
The switch appears only on marketplaces that can send us orders at all. See What each marketplace can sync.
Where the email goes
To your order notification address, the same one your website order notices use. It is set on your Business Profile. The message gives the marketplace, their order number, the buyer, the items and the total, so you can triage from your phone without opening the admin.
Alibris is different
Once Alibris switches your account to EDI Lite, Alibris stops sending you purchase notification emails, because they now expect your system to collect orders automatically. BookRouter's email takes their place, so this setting starts switched on for Alibris. Leaving it on is strongly recommended. See Switching Alibris to automatic orders.
One email per order
The email is sent once, when the order first arrives. Later updates to the same order, and re-checks of the same file, never send another. If an order is missing from your inbox, check the order is actually in Marketplace Orders first: no order downloaded means no email to send.