Could the word order here be 'Kdo těch šedesát knih koupil?' ?
What's the difference between kupoval and koupil?
Koupil is perfective, that means he bought it once at a certain point in time. Kupoval is imperfective, mostly implying repeated action.
why are we using the genitive case here
koupím něco / něčeho ?
shouldn't it be: kdo koupil ty šedesát knih?
Numbers above 4 are followed by genitive. The demonstrative is considered to expand the word "knih" hence it agrees in the case.