This is the 953rd most frequent Dutch word.
koop
"Koop" means "purchase" or "buy" in English.
Here, 'koop' is the first person singular form of the verb 'kopen', meaning 'to buy'.
Ik koop een boek.
I am buying a book.
In this case, 'koop' is a noun meaning 'purchase' or 'sale'.
Dit huis is te koop.
This house is for sale.
Here, 'koop' functions as a noun referring to the act of purchasing.