A bid price is the highest price at which a buyer is willing to buy a particular security. This may come from a market maker or be the highest price on a buy limit order in the order book. Which it is depends on the trading system used. This, in turn, depends on the market, and sometimes (as in London) on the particular security.
In general, the bid price is also the best price at which it is possible to sell a particular security - however there will be a limit on the quantity that is available at the bid price.
If trading is with a market maker, then trading a larger quantity may require contacting the market maker (or several) by telephone and asking for a price for a larger quantity. Special arrangements may also allow better prices to be obtained even on smaller quantities.
If trading is matched bargain, then the seller has a choice of accepting whatever lower prices are available for the excess quantity, or placing a limit order and hoping it will be matched.