When you search in the dictionary, you get the dictionary form. Dictionary form isn't the same as
desu/masu form.
Most students learn "To buy" in dictionary form as かう. The
desu/masu form you're talking about will be かいます/かいません respectively.
To put things into perspective, "to eat" is たべます, but if you look for it in the dictionary, you'll most likely see it listed as たべる (the dictionary form).
Later on in your studies, you'll learn how to conjugate the verbs from dictionary to
desu/masu form and vice-versa. If you need help conjugating verbs, there are a lot of resources on this forum!
Hope that helped!