[Kanbab]

Buyers and sellers need to agree on a price, it is not always the case that they agree for the listed price, hence bid and ask prices are different. For example, when thousands of people want to sell at $1000, then there will be downward pressure on the bitcoin. When someone tries to sell out $1,000,000 worth of bitcoin, he will need to find a buyer that is willing to part with $1,000,000 USD. Good luck!

[joezydeco]

...or 200 buyers willing to part with $5,000. Exchanges can aggregate orders and don't need a 1:1 match. Stock exchanges do this all the time.