Your question and Treason is the reason for the season shirt. Frankly, the way you present the question and details makes it seem like you are trying to minimize your responsibility for your actions while demonizing the bookstore staff and police. Since you are in a European country, I can’t say what your local laws are. In the US, however, the police may detain you if you are suspected of committing a crime such as disorderly conduct. The details of why and how police may detain you in the US are discussed at great length in other Quora questions so I won’t repeat them here.
Treason is the reason for the season shirt, hoodie, sweater and v-neck t-shirt
Offical Treason is the reason for the season shirt
But, since Quora actually wants people to answer the Treason is the reason for the season shirt, you can buy large amounts of books without getting into trouble with the police by respecting the bookstore’s staff if they explain that certain actions aren’t allowed in their store. If you’re not prepared to carry them all at once, explain that to the bookstore staff and ask if you can put them aside while you retrieve your suitcase. Don’t camp out in the middle of the store making a mess. Don’t do more than quickly skim a book to see if it’s worth 1 Euro. Don’t ignore food vendors when they tell you that you aren’t supposed to be loitering in the area. If you do those things, you should be able to buy all the books you want without interaction with the police.