Not true. At my bookstore, I can easily scan a barcode for a product that’s not in my database. My point-of-sale system will convert the EAN code into an ISBN and display the ISBN along with a message saying that the book isn’t in the system. That tells you whether the barcode is built correctly.
As a side comment, if you’re using an inkjet printer, make sure to print the barcode a little bigger than you normally would, and use high-quality paper. Otherwise, the bleeding of the ink can make the code unreadable. I discovered this when I printed a bunch of barcode labels on my old HP DeskJet and none of them will scan. The same labels printed on a newer printer at the highest print quality worked much better. Laser printing works almost all the time.
Also not true. You can rent several different lists, which provide the ISBN, title, author, and other information (including prices). Probably the most frequently used comes from Ingram, a massive book distributor that most bookstores buy from. Stores can obviously change the prices, but this doesn’t require changing the barcode–a price sticker is adequate, just to show the consumers what they’ll be paying.