1. They leave you alone in there
The owner of a second hand bookshop will not follow you around, assessing if you are the type who is about to spend a fortune on their goods before asking you if you 'need help' to spend your money. Sometimes they will ignore you completely. This is a rather beautiful experience in my opinion. You can spend a very long time in a second hand bookshop without feeling in the slightest bit conspicuous.
2. The Derren Brown factor
There is a kind game I play in second hand bookshops, especially in the type of shop where the books are in double rows and you have to remove piles of books to find the ones lurking behind. If you are looking for a certain book eventually you get this feeling like if you think hard enough about the title of the author it will soon show up as if it was waiting there for you the whole time. The times when this works will be worth an entire afternoon of searching and you can walk out of the store feeling like a Jedi knight AND you'll have the book too! What could be better?
3. Everyone wins
When you find said sought-after book you feel like it is worth a lot of money due to the (physical but, remember, also mental) effort you have put into its discovery. Look inside the cover- it only costs £1.20! You might feel slightly guilty as you bring it to the counter, as if you ought to pay more, only to be told you can have it for a quid. Second hand bookshops don't make a lot of money. They are delighted with your custom! If you still feel bad, pay the full asking price! Everybody wins!