As you may or may not know, we sell comics as well as games and every so often, someone comes in, expresses an interest in a comic but then says "Well, I'll just wait for the graphic novel" which is the common term for a collection of issues of a mini series or a arc, typically six issues, of an ongoing series.
Prime examples of this are the ongoing books The Walking Dead and Saga, both of which are extremely good and long running series. We used to sell 10-20 copies of each issue but now the numbers have dropped to 3 to 6. The reason is the regular collection of the story arcs, which have trained readers to wait for the trade collection, rather than picking up individual issues.
This is fine for strong series such as Walking Dead and Saga, which have a loyal fan base, but much harder for just starting series such as Curse Words and Hillbilly, which are extremely well written but may have a hard time attracting attention and generating those single issue sales needed to justify a collection.
With us, if we don't see issues of a series selling, it is hard to justify purchasing the trade collection as a lack of sales indicates not much interest in the book and, unlike mass merchants such as Wal-mart and B&N, once we buy a book, we cannot return it for credit. It stays on the shelves, just like our games, until it sells or we clearance it out. Needless to say, we work to minimize buying books that won't sell quickly.