There's a rainbow of different kinds of Mexican mercados. However, a few characteristics unite them. For instance, most Mexican mercados announce themselves by degree.
You're walking down a regular street peopled with regular folks, seeing regular stores with typical window displays and typical merchandise. But then you notice that more and more people around you are carrying bags filled with fruits and vegetables. Stores along the street may also change in character. Instead of being fronted with standard doors and large display windows, now their entire fronts may be open, with all kinds of merchandise avalanching onto the sidewalk. This causes the street to feel less formal, and to look more colorful. The mercado's congenial unpretentiousness, then, is contagious, and diffuses into the general neighborhood around it.
Right around the mercado it starts getting crowded. People slow down and gawk at this and that, and maybe even buy a taco to nibble on. Street traffic clogs up and jaywalkers outnumber sidewalk walkers. Now you smell the most wide-ranging and penetrating of mercado odors -- the green aroma of celery, and the unctuous odor of the meat stalls' unrefrigerated, dismembered flesh.
Typically, sidewalks immediately around mercados are crowded with small displays of merchandise. One spot may be occupied by an old Indian lady selling tamales from a covered pot, the next by a little girl presiding over dozens of cheap plastic sandals, the next by a young man with transistor radios and batteries, and on it goes. Maybe the goods are displayed on wooden trays about the size of a door, and set atop wooden horses, or possibly the trays are suitcase-size, with their own fold-down legs. The most humble and usually the most numerous displays, especially where Indians constitute part of the population, are those arranged atop straw mats, tablecloths, towels, shawls, or maybe nothing, on the sidewalk.