Is it assur to walk into a seforim store, look something up in a sefer and then just walk out without having ever had intent to buy the sefer (meaning – get benefit from the sefer which you never even intended to posses and the store is clearly not a library)?
As you note, a Sefarim store is not a library. The seller places the sefarim for people to see and consider to buy; they are not there for the perusal of those who certainly have no intention of buying the book.
However, in general people who wish to check something in a book might, after checking something in the book, be led to purchase it (for instance, after noticing the nice print, etc.).
Therefore, if a person is 100% certain that he won’t buy any books (for instance, he has no money, no room on shelves, etc.) it seems that this will be forbidden. There is also a concern for onaas devarim: The person gives the impression that he might but a book, but he in fact has no such intention and there is no such possibility.
However, if there is some possibility that he might but the book, even if the chance is slight, it is permitted. The seller wishes to attract such people, in the hope that they will ultimately make the purchase.