And if not, does anyone know why not?

You can post them for credit in the ARC thread in the Book Bazaar forum, but you cannot post them directly to the system.  

Here's the stance on ARC's from the help docs:

An ARC  (Advanced Reader Copy)  is an advance release of a book, usually sent to reviewers before the final copy of the book is published.

• Advance Reader Copies are also known as Review Copies, or Uncorrected Proofs.
• They are softcover books and often carry the same ISBN as the hardcover first-edition that is released later.
• On the cover, they will be marked 'Advanced Reader Copy' or 'Uncorrected Proof' or 'Review Copy' and will usually also say 'Not for Sale'.

ARCs cannot be Posted to a PBS Bookshelf for swapping, since the ARC cannot be depended upon to contain the same story/text as the published book.

• If you send an ARC, the requestor will probably contact you to ask for the credit back.
  • You will need to do this.
  • You are asked to confirm for each book you Post that it is not an ARC.  If you posted an ARC you broke the rules. 
• If you receive an ARC, you should mark it received as a Wrong Book and send a message to the sender asking for your credit back.
  • You should NOT repost an ARC (or any problem book) if you receive one.
• The receiver of an ARC is not required to return the item; she or he may agree to do so if the sender sends postage for the item's return.  This is the case with all problem books: the requestor receiving them is not obligated to spend postage to return them.

You can't post them, but you can offer them for a credit in the book bazaar.

eta: Mary, looks like we were posting at the same time!  :-)

Thanks.

Thanks for asking this question...I think the more we can get this information out there the better, since ARCs are much more readily available now than they used to be. I rarely have a problem swap, but the last three I've had have all been because the person posted an ARC of a book I had wishlisted. I get a lot of ARCs already from several book sites, and have a hard time getting rid of them, so I don't WANT any more of them. Please, please, please...offer them in the bazaar but do NOT post them to your shelf!

Cheryl

I just got the Concubine's Daughter today at Goodwill for 0.75, I bought it to read myself, no loss if I can't trade it.

Yeah, with 48 wishers you shouldn't have a problem trading it for a credit in the forums, even if it is an ARC.  Just make sure you list it in the Book Bazaar or the Historical Fiction forum and you should be good to go.