I had the good fortune of finding two hardback copies of "The Lost Symbol" by Dan Brown in the Goodwill. As 425 people want this book, I was very excited to post them on PBS. However, I can't post two copies of the same book. Currently, the one I did post is waiting for a response on the 48 hour Wish Hold. I wont know when I can post the next copy for a few more days.

Why can't two or more copies of the same book be posted?

To give everyone a fair chance at getting their books ordered.  Also probably to keep people from accidentally posting books twice when they only have 1.  You can post the 2nd book as soon as you mark the 1st one mailed.  If you want to combine them into 1 post office trip-just print the lable for the 1st one right away-mark it mailed and then post the 2nd one.  You can send a PM to the 1st person just as an FYI that you had 2 copies and marked theirs mailed early to be able to post the 2nd 1 and mail them together. 

It's generally considered to be protection against members accidentally multiple postings when they actually only have one copy to offer.  In all honesty, this scenario is probably more likely than the few times members actually have multiple copies to post. 

Maybe so that a bookseller can't dump extra copies, too.

The weird thing is, it's still possible to accidentally post multiple times. Has happened to me.