There are 4 sorts of MP3 audiobooks being offered for sale these days.  Two of them are perfectly legal, but the other 2 are not.

1. Legal: MP3 audiobooks on CD-ROMs purchased through legal outlets (bookstores, on-line stores) or here on eBay, etc., can be sold to another because when you buy the book on a CD-ROM, you own the actual property and can legally sell it to another.
2. Legal: Books that have been copied from Audiovox-dot-com can be copied to a CD-ROM and sold on eBay.  Or, you can go straight to Audiovox and download that book for yourself, cutting out the middle man, and get the book for free.  These only include older books which are in the public domain (copyrights not current).
3. NOT LEGAL: IF you purchase a download of a copyrighted book to your computer or MP3 listening device, such as an IPOD, that group of data bits belong to you alone and cannot be re-sold. 
4. NOT LEGAL: Pirating audiobooks by taking books on CD-ROM, which are often available at a public library, converting them to MP3 files, then selling those books here on eBay at a high markup is totally illegal.  Sellers often bundle together a list of books by a single best-selling author and then sell the MP3 versions for a large profit.  Or a seller might sell MP3 books from a public domain source like Audiovox, but mix in one or two copyrighted audiobooks, thus pushing their prices up as buyers see the best sellers at a "good" price.

I have investigated several instances of sellers listing pirated audiobooks in MP3 format, which have been copied onto CD-ROM and are delivered that way to the buyer.  These sellers often slip under the eBay radar, but if they are discovered, I have noticed that eBay cancels the sales and kicks out the seller.

Unfortunately, it's very easy to re-join eBay under the guise of a different seller ID, and these same audiobooks are listed again and again.  Authors like Patricia Corwell, James Patterson, Sue Grafton, etc., are basically having their work and profits stolen from them.  As an author myself, I want to see this practice stopped.

If you find an MP3 audiobook listing that contains one or a bundle of titles, but there is no photograph for that listing, and no details explaining the source of the book (Brilliance Audio, Audiovox,etc.) then be aware that the item is probably a pirated version.  If you find a listing that uses the term "mp3compactdiscs" in the listing, those books are pirated.  And if the seller lists a recent or bestselling author, then be aware, that material is copyrighted and cannot be re-sold.

PLEASE do not contribute to the pirating of intellectual material here on eBay by giving in and purchasing these books.  No matter what the seller's feedback may say, the copies of books they are selling are illegal, and should be reported to eBay using the "Report This Item" link on the item page.  I spend about 1 hour a week reporting such items with mixed results.

The seller, the buyer and eBay are all complicit in these transactions.  It is the same as pirating music on-line.  Please - just don't do it!