There are 3 places in Krakow where you can buy fresh vegetables and fruit.
Stary Kleparz registered to the history of the city. The market was located on main roads of trade in Europe. If you lived in that time, there was a specific market with many necessary products and goods. Only there you had a guarantee you would get what you wanted. Now, You can admire embroidered merchant's stall at Stary Kleparz with a picture of Divine Mother of Immaculate Conception with a child on her hand - a patron of merchants.
Nowadays, People who traded are fourth generation of merchants. They will always remember about poors and weakers.
Breaking news: it's started a strawberries season, please come to Stary Kleparz and buy it for a strawberry pie :)
Nowy Kleparz is situated on Długa Street. It's a similar place to Stary Kleparz but with different location.
Unfortunately, due to reorganization of public transport, it's difficult to reach a market. On sundays, you can exchange your book or something else for another goods without paying special fee like merchants usually do in such places. It's a great opportunity to have something interesting for your own. There is a hard time for merchants from Nowy Kleparz, because as you know, there are opened shops like Biedronka or Carrefour and so on. In connetion with this people prefer going to that shops which are nearer than e.g. stalls. Many people have moved from there to the central part of city too.
Plac Imbramowski is a market where people vitis rarely. It's probably because of the location. One of the advantage is that the market is larger than Stary Kleparz or Nowy Kleparz. There is much larger selection of products you might need.
In my opinion, it is better to buy fresh vegetables and fruit from a market than from supermarkets where a price is usually higher and probably food stuck on the shells for long time. Market's advantage is that you can agree on a price of what you want to, contrary to the supermarkets where level of prices is already agreed.