How can a book I ordered have a tracking number  (with DC) when the sender says it won't be mailed til 6 FEB?  

They finally accepted the order on 1 FEB after posting this WL book several days earlier. I've been eagerly watching for when it would be accepted and mailed because it's for my daughter for a college class.

Sounds like they printed the label with PBS postage.

I have one currently showing this way on my account page on the "Books I've Requested" tab:

Requested On: 1/27/17 2:03 AM ET 
Printed On: 1/30/2017 2:02 PM ET 
Status: Accepted - Printed 

But when I click on the "Details" button, it shows several scans en route, so she clearly forgot to mark it mailed.

Yes, tracking numbers are generated whenever a mailing label is created whether using Paypal Postage or PBS postage. When you print the label it asks what day you want to mail.

We wouldn't be able to see the tracking # here unless the sender used PBS postage though.

I messaged my sender to let her know she forgot to mark mine mailed, and, although she ignored my PM, she did mark my book mailed. Now I will be able to mark it received.

Sometimes getting the whole process accomplished can take a few days. I will often pull the book to make sure I can find it, get interrupted, get back to accepting the request....get interrupted, print the label quickly so it goes inside the book to hold it for when I have time a couple of days later to wrap/ship.  I will usually set the mail day a few days out just to err on the safe side!! 

Ok, I was thinking if they printed the label  on first, it would show as mailed on the first, then It wouldn't match if they actually are waiting to mail it on 6th.

I usually choose the latest date for my mail date but always mail sooner; I rarely change the mailing date when I make the label, so the tracking number is set for a future date and then the dates get really out of sync. Here's one where you can see I selected a 1/11 mail-by but then printed on 1/5 so the package was actually delivered before the shipping info registered.

Delivery Confirmation Events

1. 1/5/2017 6:37 PM ET - Label Created - VALRICO, FL
2. 1/6/2017 12:44 PM ET - Acceptance - TAMPA, FL
3. 1/6/2017 8:05 PM ET - - TAMPA, FL
4. 1/8/2017 8:39 AM ET - Enroute - JACKSONVILLE, FL
5. 1/8/2017 1:18 PM ET - - JACKSONVILLE, FL
6. 1/8/2017 3:18 PM ET - - JACKSONVILLE, FL
7. 1/9/2017 11:54 AM ET - - ALLEN PARK, MI
8. 1/9/2017 12:10 PM ET - - ALLEN PARK, MI
9. 1/9/2017 1:54 PM ET - Enroute - ALLEN PARK, MI
10. 1/9/2017 4:17 PM ET - - ALLEN PARK, MI
11. 1/9/2017 4:30 PM ET - - ALLEN PARK, MI
12. 1/9/2017 7:08 PM ET - - GRAND RAPIDS, MI
13. 1/9/2017 9:08 PM ET - - GRAND RAPIDS, MI
14. 1/10/2017 12:12 AM ET - - GRAND RAPIDS, MI
15. 1/10/2017 12:16 AM ET - - GRAND RAPIDS, MI
16. 1/10/2017 1:31 AM ET - Enroute - GRAND RAPIDS, MI
17. 1/10/2017 2:28 AM ET - - GRAND RAPIDS, MI
18. 1/10/2017 2:34 AM ET - - GRAND RAPIDS, MI
19. 1/10/2017 4:43 AM ET - Arrival at Unit - MUSKEGON, MI
20. 1/10/2017 8:52 AM ET - Sorting / Processing Complete - MUSKEGON, MI
21. 1/10/2017 11:45 AM ET - Delivered - MUSKEGON, MI
22. 1/11/2017 6:13 AM ET - Electronic Shipping Info Received - VALRICO, FL

Thanks for the example, Linda!

I recently received a book I couldn't mark received as it hadn't been marked mailed. I sent the sender a PM and days later she marked it mailed. She'd been on her way out of town when she mailed it and forgot to mark it.