I cannot go along with such generalisation, for general or specialised second hand bookshops. That is not my experience over the last 30 years, dealing with s/h bookshops here in Oz and overseas.
In fact, over about a decade or so through the 70's, I had a very rewarding and pleasurable relationship with the proprietor of (I think) Bohemia Bookshop in Hastings UK. It was very '84 Charing Cross Road' , Frank knew his stuff, wrote chatty, informative and helpful letters, and sought out stuff for me. As a result, I have a great number of books I may not have got otherwise, or as soon. In return, Frank ended up with a goodly amount of my disposable income, and yes, I was always happy with his prices.
About a year or so after I first contacted him, he wrote me about one of his other Australian clients, who had just visited him and who was very involved in resoration and aviation history. (Small world dept - turned out my wife worked with his

What are other people's experiences?
Has the internet and ebay done in the 'friendly helpful second hand bookseller'?