If you can't buy it here, you don't need it.  The Abastos is said to be the largest outdoor market in Mexico.  It is actually part inside and part outside -- 4 acres in total.  The market is open every day, but Saturday is the BIG day and the aisles get very crowded.  The hundreds of stalls cover everything under the sun:   fresh herbs and beautiful fresh vegetables, butchers, mole, coffee,spices, cheese, hardware, shoes and sandals, local crafts of all types (lots of rugs), major agricultural supplies, fuel, hardware, pets, housewares, clothes, live turkeys, paper products.  They have someone selling just about anything to do with life in Mexico.  One could spend many hours here.  It can get very hot in the closed in market areas in Oaxaca and the villages and some who are not used to the smell of unrefrigerated hanging raw meat can become nauseous.  The sugared baked goods can also attract a large number of yellow-jackets so those who are allergic to stings should be careful.  Many of the goods sold, e.g. clothing, shoes, bags, are not of very good quality at the Abastos.  It is a convenient spot to purchase some of the black or green pottery Oaxaca is known for if you are not traveling to the local outlying towns where they are made.  There is a long street at the western end of the market where the crafts are sold from winding lanes of stalls.

People come from all the surrounding countryside and towns to do business.  The smaller outlying town markets are friendlier and slower paced, here the emphasis is on doing business.  Bargain for everything and don't be hesitant to counter with a LOW price, because they will start with a high one.  That's how it is here and that's part of the fun of shopping.  If you don't bargain the people won't understand what you are doing.

After wearing yourself out, you can sit and have a good typical meal at one of the many comedors that are all grouped together in one central area around a kind of courtyard.  The crowds can make me feel a bit claustrophobic and the little open space is a relief.  Good local chow, hot chocolate, Cokes, etc. 

The market is a little far to walk from the zocalo, but the walk is managable and safe and passes several interesting churches.  The taxi should only be 30 pesos tops.  The second class bus station is across the street and there are crowds of collectivos in the side streets.  You certainly have to have some Spanish to navigate the confusion outside, but if you need a souvenir or a tractor part, a coffee pot or 100 kilos of seed corn, you can get it at the Mercado de Abastos.   For more costly, but usually better quality goods, the markets closer to the Zocolo are better.  More English would be helpful there and the stall owners are very helpful when making purchases of roasted chilis or mole powders or pastes  to take home.