He lived in his store under the Beis Medrash of the Maharshal and was extremely reclusive, never really spoke to anyone. One night the Maharshal awoke and found this grocer learning a deep sugya and delving into it, giving his opinion on each thing.
The next morning he sent him another deep question in gemara and he responded that he’s an ignoramus and he doesn’t understand what the rabbis are talking about. Eventually he cracked under pressure and explained that matter as well, displaying wondrous wisdom and erudition, but he pleaded that his knowledge remain a secret.
When the Maharshal passed away he instructed his congregation that this grocer be made his successor, ‘since there is none like him’. This request too he initially refused but finally acceded to.
Paraphrased from שם הגדולים מערכת גדולים ערך ר’ אברהם מוכר ירקות (אות א סימן עו).