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Where do traders take their prices data from? How can it be different from their brokers'?
To add a bit to Daniel Anderson's great answer, if you want to 'peek' at what a the set of bid and ask spreads looks like, the otc market page could be interesting (NOTE:  I'm NOT recommending that you trade Over The Counter.  Many of these stocks are amusingly scary):  http://www.otcmarkets.com/stock/ACBFF/quote You can see market makers essentially offering to buy or sell blocks of stock at a variety of prices.