。
She's a great haggler.
Literal
She [topic-は] haggling [nominalizer-の] [subject-が] is-skilled [explanatory-んだ].
Two key patterns. ~のがうまい takes a verb, nominalizes it with の, and marks it as the thing one is good at — 料理を作るのがうまい ('skilled at cooking'). ~んだ at the end is the explanatory のです contracted in casual speech: it frames the statement as informative rather than just neutral, often translatable as 'you see / the thing is.' 値切る is the productive verb 'to haggle, to bargain down a price' — culturally relevant in flea markets and in some Asian travel contexts, less common in everyday Japanese retail.