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.
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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.)
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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.