Last weeks Chum Bucket query results had a category called "Can't sell this." Several commenters asked for clarification.
For ME (thus do not apply this to what other agents do or think) it means the book may be perfectly delightful BUT I don't know who I'd sell it to.
This can happen a couple of different ways: I may have tried to sell books like this in the past and come up short; I don't know any editors who are buying this kind of book; and/or the only publishers taking on this kind of book don't pay enough for it to be profitable for us.
It's a subcategory of "Not for me" but it's not really the book, it's the market. Really good books, books that I love, can end up in this category and it gnaws me to no end when that happens.