I just did an audio to text exercise for this and I thought cebule as onionS. Then I remembered the common practice of not nasalizing the ę. I guess Polish people can't tell in this situation if they're buying one onion or more onions UNLESS they put an adjective before it to distinguish plural or singular here
Or unless they nasalize it more. But you're right, disabled the audio exercises.
Can the singular cebulę here be used in a collective sense to mean onions?