Saga trades are currently doing very well.

Marvel's Star Wars trades perhaps? There is a movie coming up plus they have some solid creative teams on them.

Walking Dead is probably the biggest mover in trades form one of the big 5 publishers currently. Saga also does well and got attention form its Hugo nominations.

The should look into Bookscan and what services they provide, it is the leading compiler of sales data for the book trade outside Diamond.

As others have said, a lot depends on the demographics of their customer base, but if it is tourist heavy, I would look at a sampling of volume 1 or complete OGN that someone would buy to read on a train or plane or in a hotel room while travelling rather than deep runs of series for repeat customers. If the customer base is transient due to the tourist trade, it's less likely someone who picks up volume 1 of a series will be back for volume 2. If a series catches on with a local customer, you can always order later volumes in a series for them, but to tie up operating capital in those volumes to sit on a shelf is risky at best. If it is a tourist/transient customer base, I would also avoid high end hardcover collections, omnibus, etc. as people on the go will not likely buy those to haul around with them. Format may almost be as important as content in sales to that particular customer base. It might be worthwhile for them to look into what types of product airport bookshops are carrying as well to see what kind of trades/graphic novels if any they stock and have success with. They might even look at the output of some of the publishers who specialize in the booktrade and not the direct market, like FirstSecond Publishing.

-M

One more thing I thought about on regard to the "tourist area" aspect of this discussion is: what type of tourists are we talking about?

Are these older empty-nesters out on holiday, as it might not make sense to have any comic-related material?

Are these families with younger children, for whom Spider-Man and Squirrel Girl might be more appropriate?

Are they millenials, who probably don't want physical copies of the books, as they would be more apt to go digital?

So many variables to consider, but I would think that having the attitude of "people like comic book heroes and stories, so we should stock those things" might be a recipe for disaster.

Aahhh.. I didn't realize you were not in the states. In that case, especially since it's tourists.. I might suggest more European comics.. Tintin, Asterix, Valerian, Judge Dredd, etc.