Depending on when you go this market transforms itself
On Mondays its a lively flea market
Early on Saturday morning its a bird market, caged birds, rabbits etc
Early to mid Sat afternoon a farmers market is bustling with organic produce, herbs etc
The Noordermarkt is a great spot to shop on Saturdays and Mondays.
The Saturday street market is a combination of food (many bio food stalls) and second hand stuff (from Brick-a-Brack to some antiques).
The Monday market is a more regular market with fixed dealers, the salesman on the Saturday can vary from week to week, what makes the Saturday market more attractive.
The Noordermarkt is almost the oldest Amsterdam street market; it started in 1618. It's a shame that such a lively place is rather unknown to the many tourists that visit downtown Amsterdam.
What to buy: Whatever suits you.
I love going to this market when in A'Dam. It is kind of sequestered in a corner of the Jordaan but a nice walk on a saturday morning. I have seen birds, cheeses, furniture, antiques, lots of other good food and fun items to look through, and is not quite as intimidating as the Waterlooplein flea market. There is a small cafe (cant remember the name) right at the market where you can get a cappuchino and a bakery item and watch the crowd come in---great people watching spot! The best market is, in my opinion, on saturday mornings.
What to buy: Cheese, antiques, birds
What to pay: good prices for good food
The Jordaan area of Amsterdam is a joy to explore. Almost every street in this local area still has it's own bakery and flower shop.
And there are markets too:the Noordermarkt on Mondays and Saturday and the super general market on the Lindengracht on Saturday morning.And not forgeting the local Farmers market for organic produce,around the Noorderkerk on Saturdays.
What to buy: Anything and everything is available at the local markets and the small streets of the Jordaan have many specialist shops such as hatters and candle makers.