Read the law and I'm not seeing where this situation violates Florida law in regard to a transaction broker.
It violates the law because the broker's responsibility is to close the transaction. As soon as they start giving preferential treatment to specific buyers they are then acting as an agent of the buyer and not the transaction. It's a conflict of interest.
Why shouldn't the first prospective buyer hold an advantage over subsequent prospective buyers as they were in an active negotiation with the seller? If I were the seller I would have had the agent go back to the other prospective buyers in order and said: "There is a new offer on the table, If you are still interested the price is now y+1" No laws being broken here, just doing the right thing.
The reason the first prospective buyer shouldn't hold an advantage is because this is business, not the schoolyard. Calling "I saw it first" doesn't work in this case. But you do make a good point about what would happen if you were the seller. Remember, as a transaction broker, the broker is not responsible to represent the interests of any one specific buyer and they are also not responsible for getting the seller the maximum price possible. All they are responsible to do is present all offers to the seller. If I were the seller I would ask my broker to contact all interested parties to try to create a bidding war situation (something I am guessing brokers try to avoid because it can get ugly). I think we're going to have to agree to disagree on this point of what is "right".
With all due respect, ELMC, I don't read marynvince's post as wanting to be beneficial to herself and I agree with the question she has raised. When someone makes an offer, it may simply be a starting point. In OP's case, the seller made them a counter offer, which is essentially a verbal contract saying they will sell it to the buyer for the counter offer price. It does not then seem ethical for the seller to then walk away before receiving an answer and accept someone else's offer. I don't see why first come first served can't apply and I don't see how this would be unfair to anyone.
First off, I have to say that I really appreciate the tone of these conversations and the fact that we are able to disagree and be polite at the same time.

I hope I wasn't being accusatory when I said that marynvince wanted the rules to benefit her, because that wasn't my intent.
Anyway, to give you my thoughts on your comments...the problem with this situation is that it isn't as cut and dry as we try to make it sound. In Mary's case, she proposes a situation where there are two buyers and the seller makes a counter offer to both. Buyer #2 accepts the counter offer but if things went the way she suggested, the seller would need to wait to hear back from Buyer #1. Let's say that it takes half a day or so to respond and Buyer #1 does not accept the counter offer. So the broker goes back to Buyer #2 who has moved on and purchased another contract. Now the seller is left either without a buyer or is forced to take a lower price. Furthermore, this adds to the time it takes for the contract to close, so the broker is not fulfilling his duties to the contract in the best way possible. This is just one example of the many things that can go wrong.
As an aside, although it sounds nice, what you are suggesting would create an extremely difficult situation for the brokers. I have been involved in bidding on a contract that had at least three bidders (but I suspect more than that). So not only does the broker have to keep track of all the bids, but they also have to track the chronology of the bidders and communicate with each of them in the proper order during every step of the offer/counter offer process. That would be completely unmanageable.
Listen, as a buyer I would love for your suggestions to be the way it is. I have done exactly what you said...offered a low starting bid in the interests of opening negotiations...only to never hear back. What has happened in the meantime was that someone else came in and made an offer higher than mine that was accepted. The broker I use was under no obligation to let me know that I was outbid, and so she didn't. I didn't get the contract. Did I feel bad about that? Sure I did. But it's the way it goes. I would much rather accept that than being denied the opportunity to purchase a contract because someone called dibs.
I guess what I'm trying to say is that I see your point, and in theory it would be a good thing...but only for buyers (specifically the first one in). It would be worse for sellers, worse for brokers, and I'm not even sure it would work in real life the way it has been laid out here in theory.
But as always, I could be wrong. Thanks for your thoughts!
