You can not buy at bid price, no matter what; until it is move to ask price
Arthur, consider the following scenarios.
XYZ trades on NASDAQ, 1,000 bid at 10 and 1,000 offered at 10 1/4. Gary places an order to buy 500 at 10 1/16. Under the new SEC rules, the market maker that receives this order must represent it to the market, with a bid for 500 at 10 1/16. Now Gary's order is the "bid". If another customer, anywhere in the world, wants to sell 700 at the market, the first 500 go to Gary. Gary buys at the bid. Then the bid drops back to 10, and the remaining 200 get sold at 10.
There are some potential complications. The market maker who receives Gary's order may choose to bid for 1,000 at 10 1/16. Then he display's a bid of 10 1/16 for 1,500. When they receive an order to sell 700 at the market, they are allowed to fill their own bid before they fill Gary's. Worse yet, they can hold the bid at 10 until they receive the market sell order, and just then decide that they want to bid 10 1/16, fill their own order and leave Gary unfilled.
In general, you can buy at the bid and sell at the ask, but you are not guaranteed a fill. Gary could even buy at 10, but many other trades might take place at 10 before he gets his fill.
To learn more:
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