Panjiayuan second-hand book market only opens on weekends, and there is no booth on weekdays.
I don't know much about old bookstores, but what you saw in Panjiayuan is actually quite right.
You can talk to the stall owner and ask them to help you understand.
Another option is online, such as Confucius Old Books. If you like to go to Taobao, go to Taobao.