I'm not saying that your experience did or did not happen as you described it, but I want to play devil's advocate for a quick second.
Some of the resale brokers have hundreds of contracts listed, many of which, as you mentioned, belong to other brokers. How can you be 100% positive that the other DIS Member bought your exact contract? There is a possibility that the contract they posted about was the same resort, # of points and use year, but a different contract, is there not?
Secondly, how do you know that Fidelity didn't have the other offer submitted before yours? Perhaps it might not have been fully accepted, thus still showing as "available". But perhaps Fidelity has a policy where they will let things play out with a direct client before giving the offer to a secondary broker? Sometimes it can take a few days for the back-and-forth negotiations between buyer, broker and seller to become finalized. Your offer might have fallen somewhere in the middle, and your broker may have simply mis-communicated things back to you? If it was the same contract, we're only talking about the difference of one day, which is nothing at all in the resale world.
Lastly, I will just mention that each resale broker has a number of people selling. Perhaps another person within Fidelity "sold" the contract, but neglected to update the status, whereas the other salesperson dealing with your broker wasn't aware that the contract was no longer available? This really could have been an honest mistake, rather than a deceitful act as you described it.
Without knowing all of the facts, and with no surefire way of knowing exactly what transpired and when, I really don't think it's right to flame Fidelity and trash their credibility for something that really might not have gone down as you described it.