This market is not in the city centre and it's a bit further, but still walking distance. We came by just after 9:30 on a Friday. There are fewer crowds here, probably because of its location, which makes it a prime spot for food tours. We didn't do one but we just wandered around ourselves. There are fewer options here than Mercato Centrale but the trade off is that you don't have to deal with all the people. They sell a lot of the same stuff - fresh produce, meats, and some vendors selling food. We tried the tripe sandwich here, which wasn't as good as the other market, but still delicious. We also bought some strawberries outside, which might have been some of the best strawberries I've ever eaten in my life, so definitely don't sleep on those.
Mercato di Sant'Ambrogio is worth the trek!