If you are in the Eixample area of Barcelona, enjoy the freshness of market food, and are looking for something a bit different, I suggest paying a visit to the Mercat Municipal del Ninot market for a midday meal where a number of the vendors cook and serve their own foods on site. I understand that this is the first fresh market of its type in Barcelona (only open for 2 years now) where they also serve food prepared within the market.
My favorite is a large family run butcher shop named Morales along the southern perimeter of the market. As you enter the building from the main entrance, just turn left after you are inside the market and walk to the very end where you will run right into the shop.
Melanie Morales, the daughter, a young and very friendly and enthusiastic woman, runs the restaurant part of the shop and she does all the cooking herself. She serves grilled chicken and beef, 5-6 kinds of homemade croquets (chicken, spinach, fish, etc), and serves it all together with great fresh salads. They also serve beer and other assorted beverages.
It's a really enjoyable experience for me eating in a fresh market like this, something I have grown used to living in Asia, where you know that you are being served only the finest and freshest foods.
Melanie has six tables in her shop, which are often filled up by around 1PM and serves food starting from 11AM until 4PM (closed on Sunday's).
She offers a daily menu for 11.95 Euros and a special 3 course menu for 19.95 Euros served with croquets, foie gras and a 1 kilo T-bone steak, drinks and desert.
You can alternatively make your choice of having her cook anything for you that they offer fresh for sale within their large butcher shop.
I particularly like their marinated chicken kababs that she serves with a beautiful garden salad with fresh strawberries and some homemade fried potato wedges.