Even if a home is listed for $400,000 and you offer $400,000, the seller doesn't have to sell to you. You can't force him. And your transaction is between you and the seller, not between you and the seller's Realtor.
The risk for the seller is that (depending on how the listing agreement is written) if a full-price offer comes in, then the seller may be obligated to pay the commission even if the seller declines to sell the house.
There are many instances, even today, in which a property is listed at a certain price and is sold for more. One recent example: I showed a couple a home, a foreclosure, listed for $180,000. It received 8 offers in 3 days. The accepted offer was all cash for $185,000. None of the other potential buyers (including some who offered full price) have any recourse against the seller for declining their offers.