In a nutshell, the bid is the estimated price a dealer may be expected to try to purchase the coin from another dealer if needed for stock. Purchasing from the public, the buy price would be lower. The same would be true if the dealer did not need the coin in stock.
Thus, many dealers will offer a prospective seller a percentage back of the bid price. If the dealer has a customer waiting for a particular coin and someone comes through the door, the dealer would more readily consider somewhere between bid and ask for the coin, or even going front of ask, depending on what their customer is willing to pay.
The ask price is the estimated price that a dealer may try to get from another dealer that is in need of the coin for a customer.
There are numerous sources the CDN staff utilize to make their estimations including auctions, price lists, network bid/ask levels, and other publications. Because of the plethora of sources, and the number of dealer/wholesale sources used, you should most definitely expect to pay more walking into a B&M store-front, or even on
ebay, Teletrade, etc.
However, there are those coins and series that are just not followed very closely and where the actual current market is skewed from the publications. In these cases, even the CDN can be grossly "over-priced".