If you're ever in my hometown of Portland, OR, USA, be sure to visit Powell's Books. Over 1,000,000 books spread over 68,000sq/ft / 6,300sq/m. Back in the day, they were buying over 3,000 used books/day. COVID has hosed all that, but they still have 64,000+ used books available Online.
Image Source: Conde Nest Traveler
They have a 'Rare Book Room', too.
Image Source: Powell's Books