What is the connection between St. Michael and people who work in grocery stores?

This is most likely an adoption, due to the fact that in Medieval times, soldiers (police if you like) would protect the market places where grocers would sell their goods.

G.K. Chesterton did not have a high opinion of the Grocer as the hero of modern times and would probably been led the protests against shuttering inns in favor of herding us in to grocery stores. He wrote several songs against grocers, but this one mentions St. Michael specifically:

Pardon, dear Lady, if this Christmas time, The Convalescent Bard in halting rhyme Thanks you for that great thought that still entwines, The Wicked Grocer with more wicked lines; These straggling Crayon lines - who cares for these, Who knows the difference between Chalk and Cheese.

Not wholly sound the saw, accounted sure, That weak things perish and strong things endure: Milton, six volumes on my groaning shelves, May groan till Judgement Day and please themselves, As, harsh with leaden type and leathery pride, Puritan Bards must groan at Christmas tide:

My table groans with Stilton - for a while: Paradise Found not Lost, in Milton's style Green as his Eden; as his Michael strong: But O, my friend, it will not groan there long.

Stilton and Milton - G.K. Chesterton

I believe that the notion that St. Michael as the angel who guards the Garden of Eden after the fall, and the Garden of Eden being the ultimate grocery store, conjures up the Grocer as the modern guardian of the garden.

What is the connection between St. Michael and people who work in grocery stores?

If you desire a source that states how St. Michael the Archangel became the patron saint of grocers, you are going to be very disappointed.

In France, St. Michael is one of the patron saints of France. He is also the patron saint of Protector of the Jewish people, Guardian of the Catholic Church, Vatican City, sickness, police officers, military personnel and much more.

For the French, he also the patron saint of fencers (escrimeur), pastry workers (pâtissiers) and low and behold grocers (épiciers).

Michel est aussi le patron de l'Église catholique et de la ville de Bruxelles et de plusieurs corps de métiers anciens, comme les épiciers, les escrimeurs, les manœuvriers, les policiers, les soldats, les mariniers, les boulangers, les pâtissiers, les tonneliers ou plus comme les parachutistes. - Saint Pâtron des pâtissiers

For the French, it came as second nature to invoke St. Micheal as the patron saint of grocers. He is one of the patron saint of those who are sick and their country for several centuries. The grocers of those Medieval flea markets need a patron saint and St. Michael had a great notoriety in France and it fell to this Holy Angel to protect him.

Le roi de France, Charles VII, ordonna que l'image de saint Michel décorerait la bannière royale, comme étant le gardien et l'ange tutélaire dela France, le roi Louis XI proclama le méme saint protecteur. du royaume, et fonda en son honneur l'ordre célèbre de Saint-Michel. Dans les anciennes corporations des métiers, saint Michel était le patron des pâtissiers, sans doute parce que les ouvriers commençaient leurs veillées ** 29 septembre, jour de la fête du saint, et que ce jour-là un pâté était le mets traditionnel offert par le maître à ses ouvriers**. - Origine du prénom Michel

That right on the Feast of St. Micheal, in olden times, workers received traditional eateries in his honour!

Yes, those going to Medieval flea markets, the faithful needed protecting. Sanctuaries to St. Michael were built by the faithful in the 4th century, when he was first seen as a healing angel. Over time his role became one of a protector and the leader of the army of God against the forces of evil.

A flea market (or swap meet) is a type of bazaar that rents space to people who want to sell or barter merchandise. Used goods, low quality items, and high quality items such as collectibles and antiques are commonly sold. Many markets offer fresh produce or baked goods, plants from local farms and vintage clothes. Wikipedia

The word "flea", which is "a small wingless jumping insect that feeds on the blood of mammals and birds. It sometimes transmits diseases through its bite, including plague and myxomatosis."

Do people mean “flea market” is like the dirty market which has a lot of fleas?

The logical choice for the patron saint of grocers, historically seems to be the Prince of the Heavenly Hosts, St. Michael the Archangel himself.