also says not for resale. Are these not allowed?

ARCs are not allowed to be swapped on the site at all.

From Help Center:

ARCs must NOT EVER be offered here, not in official nor unofficial swaps. They are absolutely prohibited, since

1. the ARC cannot be depended upon to contain the same story/text as the published book and

2. these items were given free to reviewers; they are not actual books.  The author and publisher did NOT get paid for the ARC and we do not support cheating authors/publishers. Posting an ARC is strictly prohibited, and ARCs may also NOT be swapped "unofficially".

If you mistakenly DO send an ARC, the requestor will probably contact you to ask for the credit back.

• You will need to do this. You can use the Give Refund button on the transaction in your Transaction Archive (after the requestor marks the book received with a problem/wrong book)
• You are asked to confirm, for each book you Post, that it is not an ARC. (So, if you Posted an ARC you broke the rules.)
• If you post an ARC after being told about the rules, you jeopardize your PaperBackSwap membership.
• 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 should destroy/recycle the item.
  • Some publishers are OK if you offer ARCs as a giveaway on a blog or other book-promoting social media account -- since ARCs are intended to promote the book. Not all are Ok with this though. You can check with the individual publisher.
• 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.  The receiver of an ARC should destroy/recycle the item. As with all problem books: the requestor receiving them is not obligated to spend postage to return them

Someone didn't check closely enough I think, Charles, or they did it anyway thinking they might sneak it past you.  I seem to find a lot of people can't tell a book is an ARC or aren't very observant.

To be fair, sometimes they are clearly marked, and sometimes they are not.

I agree sometimes its hard to tell from the outside. I won an ARC from an author and If I didnt know it was one I would swear it was a regular.

I posted an ARC book without realizing it, there was no sticker on the book indicating that it was. I also got it from the free pile at my local library. Yes, I am a book nerd I get a lot of my books from the library free pile. I did refund the credit because I was embarrassed. It was a book on shelf and I never read the book. Just saying accidents do happen. I did not do it intentionally.

Must be an epidemic of ARCs, I just received on yesterday.

As for the markings, the trick is to check the back cover of the book. ARCs almost always have some publishers' notes for reviewers. The one I received also had a small circle marked review copy---also on the back cover.