Mary Barton works for Mrs. Simmond as a seamstress. While the shop is reportedly well off, Mary is paid close to nothing and worked to the bone:

...she had engaged herself as apprentice (so called, though there were no deeds or indentures to the bond) to a certain Miss Simmonds, milliner and dressmaker, in a respectable little street leading off Ardwick Green, where her business was duly announced in gold letters on a black ground, enclosed in a bird's-eye maple frame, and stuck in the front parlour window; where the workwomen were called "her young ladies;" and where Mary was to work for two years without any remuneration, on consideration of being taught the business; and where afterwards she was to dine and have tea, with a small quarterly salary (paid quarterly, because so much more genteel than by the week), a very small one, divisible into a minute weekly pittance. In summer she was to be there by six, bringing her day's meals during the first two years; in winter she was not to come till after breakfast. Her time for returning home at night must always depend upon the quantity of work Miss Simmonds had to do. (58-59)

This location is where Mary hears much of the gossip going around the city and where she learns of key events of the novel, such as Young Mr. Carson's murder, and interacts with the character Sally. 

Gaskell, Elizabeth. “Chapter III.” Mary Barton, edited by Jennifer Foster, Broadview Press Ltd., 2000, pp. 50–59.