What is the verb for guarantee?

Answer 1

Guarantee is already a verb, because it is an action.
Other verbs are guarantees, guaranteeing and guaranteed.

Some example sentences are:
"I guarantee this is the lowest price".
"He guarantees we will not find it cheaper any where else".
"He has been guaranteeing this for years".
"Although he guaranteed it was the cheapest price, it was not."

Answer 2

Guarantee is the verb.