The first mercado I ever saw was in Guadalajara, decades ago. It was housed in a three-story building and I spent three days there! I'd probably need even more time there now, since I know more about what they have to sell.
The vendedora de verduras (veggie lady) at Mercado Mazatlan. Tomatoes were three pesos per kilo. Go ahead, do the math. A peso is roughly equivalent to a dime. A kilo is roughly two pounds. (These prices were from spring, 2007)
Thanks to a blog by Jim Johnson in Mexico City, I found this list of mercados to take with me next time I go traveling. The list is part of an engrossing website, Mercados: Traditional Mexican Markets, which also includes a page with advice about eating mercado food. For those of us in our first year in Mexico, it's probably a good idea to follow this advice closely. After a year or so, apparently our intestinal flora adapts well enough that we can be braver about what we eat, and where. But I don't think I'll ever be ready to buy meat or fish that has been sitting on a counter in a mercado for hours. No, gracias.
(A friend told me about a meat vendor in a mercado who was boasting that while other stands were fly-infested, no bugs were hovering over his merchandise. Then he brandished a can of Mexican bug spray with which he proceeded to liberally douse his stock, while my friend backed cautiously away.)
Herbal baths to help with business, virility and bad luck, in a fascinating booth selling charms, herbs and incense at Mercado Mazatlan
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Here, of course, we also have tianguis, four of them in the area. Tianguis is like a mercado, only outdoors and not open daily. In fact, I have to cut this post short and get ready to leave for tianguis this morning at Empalme. The Capt is looking for a cushion for a new stool I can sit on while I sing. I'm looking for bandanas to add to my collection (a buck apiece) and some veggies for a stirfry.