books: what do you look for?

Another regular question from sellers.
I understand the motivation, but buying for the store isn't easily reducible to a useful formula.
It isn't binary, like buying for the internet, where books are either good (the lowest listed price is worth your while) or bad (there are a million copies listed for a penny each).  Books for the shop are a judgment call. These days most people get the reply I've honed to a razor's edge over the years- "stuff we can sell."

And that's basically it- if the price is right and I think we can sell it, I'll buy it.

But here, I'll take a swing at a bigger answer.

So, what do I look for?
Or rather, what factors combine to create the ethereal scent of saleability that acts as catnip on the experienced book buyer?

Condition.
Sometimes a crummy book is worth getting because it's super clean.  Clean books make your shelves look nice, even if the content is kinda sucky.  I'll take a great book in whatever shape, but we pay more for nice copies.

Scarcity
There are many, many books which we sell the ass out of that I won't buy over the counter- there are so many copies floating around out there they wash up in the shop without us having to actively seek them out.
Case in point, Nicholas Sparks.  He writes romantic potboilers, the best known (and presumably most romantic and pot-boilery) of which is The Notebook.    We sell every one of his books we get in, we have a decent selection on the shelf right now, and I can't remember the last time I bought one over the counter.
Because there are millions and millions of copies of all his books out there.  We just end up with them- people leave them, we find them in thrift stores, whatever.
This is the same dynamic with any 'bestselling' author- at any given time there's one book you'll pay for, whatever their latest release.  The rest of the bibliography you know will show up whether you want it to or not.

Topic
Plenty of subjects are the antithesis of Nicholas Sparks and his overpublished ilk- smaller print runs, limited distribution and fanatical interest from a relatively small but highly motivated demographic.  We'll by anything we see regardless of quality and almost regardless of condition on, say, Netsuke.   Or cultivating orchids. Or less specifically, eastern philosophy.  Or Art Deco.  Or the Arts & Crafts movement. Or the Spanish American War.  Or chess.
But there are so many of these small topics it's pointless to bring them up- even a half-assed list would fill a book.  And I guess someone could then memorize that book, but it goes back to Teller's rule of magic- the only person willing to put that much energy into the 'trick' would be a professional book dealer.

Author
There are plenty of authors that always sell, although it's more complicated than subjects.  You want anything at all by Hemingway.  Malcolm Lowrey has exactly one book you can sell, but you can always sell it- Under the Volcano.  Ditto for Frederic Exley's A Fans Notes, J.P. Donlevy's Ginger Man, ad nauseum.  Michael Chabon always sells.  Ayn Rand, you can sell an infinity of Atlas Shrugged & The Fountainhead, but you can't give away the rest of her stuff.  John Steinbeck always sells.  CS Lewis' nonfiction and religious writing always sells, his Perlandra series always sells, but the Narnia books fall into the "too common to pay for" category.  Etc etc etc.

Publisher
This was more meaningful before the internet.  Back then, you'd basically trust certain publishers to be selective enough that anything the put out would be worth carrying.  So MIT Press, Routledge, Princeton Architectural Press, University Books, Philosophical Press, Basic Books....there were quite a few publishers you'd just buy on sight.  Nowadays, it's easy to check individual titles and the publishers cache has been eroded.  It's still something you look for, but it doesn't carry the same weight.  About the only publisher left that's 'buy on sight' for me is Dover.   Which is mainly a reprint house dedicated to keeping 'important' out of copywrite books in print.  Their editorial eye is peerless and it's been my experience that you can't go wrong following their lead.

Those are just a few of the criteria when I consider buying a book.
Time presses, so I'll close with one last general rule of buying for a used shop, the single most useful tip I know-

The more specific & esoteric the book is, up to a point, the better.

So, a book on wine is good- we're a wine region, there are a lot of people looking for wine books, we can sell pretty much anything we get.  A wine book specifically about this area would be better.  A book on one particular area winery would be even better.  And best of all would be a viticulture book, a technical book on the specifics of growing and processing wine grapes.

You can apply this to every topic out there, and it will never lead you astray.