I have two books that I wish to post--both hardcovers. When I enter the ISBNs, the data that appears says that these books should both be paperbacks. I have seen the help item regarding entering the ISBN on a PAPERBACK when the book Listing Preview says "hardcover." However, my problem is just the opposite--a hardcover whose book Listing Preview says paperback. Should I still follow the help instructions and enter the data by hand?

Yes, you should post the book as if it has no ISBN.  

This happens because publishers use the same ISBN's for both paperback and hardcover versions of some titles. 

Yep, the books need to be entered as if they don't have ISBNs.  The reason is that the system can't handle more than one booktype per ISBN. 

I believe you now have to enter the book w/o the ISBN#.  PBS will assign a number for the book.