I will make a big exception for second-hand bookstores though. The character of a second-hand shop is very hard to replicate online, as is the opportunity for serendipity. Every online shop has a search function that beats the life out of chance finds. In a second-hand shop you may happen upon a book that has been sitting on a shelf for years (maybe even misplaced) waiting just for you. I cannot resist that siren call.
I visited the US last year. In a charming little second-hand bookstore (including a grumpy owner) in Flagstaff I chanced upon the collected works of Carol Shields. I had never heard of her but she may well turn out to be a literary love for life. Thank you, grumpy owner.