There are maybe 20 or 30 tiny shops in Asni, mostly selling exactly the same things. Food, water probably a really good idea if you're planning to head further into the mountains. Bottled water gets more expensive the further you get from Marrakech? And if you feel like being conned, there are Loads of really naff trinkets being sold by hustlers. BEWARE: Hustlers are out to get you. They'll invite you to their house, give you lots of mint tea and sell you an excellent but massively overpriced meal (50Dh each for cous-cous. I was hustled by these guys Muhammed and Muhammed who although were friendly and spoke good English, became really pushy when they got out the inevitable "silver" bangles. They spend all day waiting around the cafe opposite the petrol station. Perhaps it's just how they make a living, but it's easier to not get too involved with them. Sunglasses allow you to avoid making eye-contact.
The weekly market Saturdays is big. Traders travel for hours and hours to get there. However, remember Marrakech probably has everything you can buy here, at a lower price.