Kvatch K.
Yelp
This is a pretty typical Mexican 'mercado', hundreds of stalls selling everything from clothing to tourist trinkets, live animals, leather goods, balls of molé, street food, and just about anything else you can think of. There's little else I can say, but if you haven't been to a proper mercado, it's worth a trip just for the atmosphere. But the real reason you want to hit Mercado Benito Juárez is for that bottle of mezcal you must have before you depart Mexico.
There is no better city to sample mezcal than Oaxaca, but guided 'mezcal tours' in the local tasting rooms, or experimentation in the restaurant bars, won't help you find that perfect bottle at a reasonable price. Tasting rooms often have a few high-end brands that they have relationships with. Restaurants may have more selection, but good luck finding that wonderful Tepeztate you sampled in any of the smaller stores. This is where Mercado Benito Juárez comes in with it's 6 or 7 (by my count) liquor stalls that are stocked with mostly...you guessed it...mezcal!
Be prepared to bargain, or at least to let out a well-timed 'que caro!'. Shop around. Know the distiller's price for their own bottles. (You probably won't be able to undercut that price by much, if at all, but when the stall owner quotes you a price 50% over the distiller's price, you can shake your head and respond with a price that is fair to all.)