。
She isn't overestimating my abilities.
Literal
She [topic-は] my ability [object-を] isn't-overestimating.
買いかぶる is a vivid native compound verb meaning 'to overestimate, overrate, give too much credit to.' The first half 買い is from 買う ('to buy'); the metaphor is the buyer paying more than something is actually worth, then projected onto judging a person's abilities or character. The negative form here uses ~てはいない (a slightly emphatic, formal negative of ~ている) rather than the more casual ~ていない — a small register lift. The whole sentence functions as a compliment-via-denial: she's not blind to the speaker's abilities, but seeing them clearly.