When I see "W" or "WA" next to one of the books I have on my bookshelf that are on hold, does this mean that my copy would be the next or only copy in the system to trade if I posted that book?   

Those mean:

W = that the book is on someone else's wish list

WA = that the book is on someone else's wish list and that they have it on auto request

If folks are waiting, and you post the next one then, yes, you should be the next to trade (barring any RC issues, of course).

I'm not aware of any documentation indicating you may have the only copy available, just "you got it, someone else wants it".

Cheers,

Catt

Catt is right on most of it.

The only minor correction is that if a book is showing W or WA, then there are currently no other copies of the book with that ISBN in the system available for swap.  If another copy was available at this time, it would be offerred to the next person who has it on their wishlisted. 

Someone might have their copy on hold as you do though.  That also does not mean there are not other copies of the book with different ISBNs available.