Yes, I think that is what it is: the sense of discovery. A good used bookstore may look random but it is not, it has an organization to it, and if you master it, you find discoveries scattered among more conventional offerings. I still recall the glee I experienced at Strand Bookstore in New York in 2000 when I fell upon a copy of an old mystery book that you could not find anywhere, online or on eBay or anywhere, and it was $4.00 to boot. The book is still next-to-impossible to find, but hey world, I've got a copy of it, so there! :)