I've found that it's hard to find specific books you know you want, unless they were really popular, but it's easy to find something interesting you've never heard of before.
That, the chance to happen upon something you didn't know you wanted, is what adds to the charm of second-hand bookshops.
A few weeks ago I picked up Umberto Eco's Misreadings in the second-hand section of our local bookshop. It's a collection of short stories from the mid twentieth century translated in English in the nineties. I grabbed it for a closer look because the author was known to me (The Name of the Rose, Foucault's Pendulum), and because the back of the book introduced one of the stories therein as a pastiche of Nabokov's Lolita, wherein a certain Umberto Umberto (heh) pines for an elderly lady referred to as 'Granita' (incidentally, 'Nonita' in the Italian original).
That story was a delight to read and totally worth it.¹
I also would never have found, never mind purchased that book online. My biggest source of books is a yearly book-fair (in Deventer) where I will gladly spend hours trawling through banana boxes for cheap paperbacks and random chance finds. (Except this year due to bloody you-know-what.) So hurray for the serendipity of second-hand bookshops!
I think this very much depends on where you are. In england, culture has always been a dirty word, so second hand bookshops have the kind of books english people read, which are dismal. In germany, I've had much better luck - packed shelves of reclam-edition books, for instance.
