Bob Armstrong is a smart man. He always graded his coins conservatively, and usually most of his inventory sold to other coin dealers.
Unless you sell the very finest in top graded coins, what Bob does is a very astute business tactic. I have been on both sides of the dealer table, and I know, if I really want to sell something, I undergrade it (grading a nice AU coin as EF) and a coin dealer will snap it up even before the show opens... Some dealers price their inventory relative to price guides (which are always negotiable) and some dealers price their items to sell them...
Coins at the upper 1% of the population, with MS-65+ grades and top eye appeal, will always command a superior price. Other coins, you can usually negotiate with a dealer down to 75-80% of trends, depending on how much a key date that coin is. Don't expect a heavy discount on a nice 1889 10-cent or 1955 NSF cent. But, for common dates, like a 1939 1-cent in MS-65, you can always snag one at about 60% of trends... my advice, having been on both sides of the table, is to ask the dealer "what their best price is"... most dealers have quite a bit of wiggle room, especially with common coins that have been in their inventory for years.
Try buying coins, as an anglophone, at a coin show in Quebec... you are lucky if you can negotiate a price down to 90% of CCN Trends prices. That said, if I am picking a cherry, it is because I see something they don't - and I happily pay their price to snag it...