in uyuni there's a sunday market which i found really expensive (I stopped asking after the second doubled price I've heard), an everyday market that could be a good choice for textiles (in less "known" town prices are lower for the same stuff) placed just in front of the dull buiding in my hotel tip and in front of colca tour, and even a local market which I have no time to visit but seemed the most promising one.
What to buy: wool and fabric are the products to buy in bolivia.
What to pay: it depends on your trading skills
I know that Uyuni has a large street market on Thursdays (I have been there twice on Thursday). It may also be open on Sundays. It stretches for two blocks on Av. Potosi from the Hotel Kutimuy almost to the clock tower. It is Uyuni's equivalent of a department store and grocery store combined.
All the the altiplano towns have numerous shops that make a living selling the necessities to locals and also carry many luxury items for travelers passing through. These are small Mom & Pop operations and offer a glimpse into local life in this harsh environment.
What to buy: You can find most things you are craving like chocolate, cookies,bisquits, and soft drinks. And more importantly, beer and wine for the evening's party.
What to pay: A bit inflated compared to La Paz but really not bad considering how remote this is and by western standards still incredibly inexpensive.
Any shop - N'importe quelle boutique de bouffe.
What to buy: Buy some water, around 3-4 liters per people for the 4 days trek. Mandatory !
Achetez de l'eau, environ 3-4 litres par personne pour le trek de 4 jours. O-b-l-i-g-a-t-o-i-r-e !
What to pay: About 5-6 bolivianos (0.8 USD) per bottle
Environ 5-6 bol (0.8 USD) par bouteille