Agreed-- sounds fishy to me, too. I'm not a lawyer. I'm not a licensed real estate sales agent in my locality either, but I did complete a course of training to that end. I never heard such a thing. If a seller were discriminating against a protected class in refusing such an offer that would be illegal. Holding-out for a better offer (whatever that memans) however, AFAIK, the seller's prerogative so long as they're not discriminating against a protected class.

In belgium a written counter-offer is binding. So, say a buyer makes an offer via email, the seller makes a counter-offer via email, the buyer accepts, then the seller is obligated to sell at that price to that buyer, even if a higher offer comes in from someone else. If they refuse to sell they can theoretically be forced to pay a fine, though in practice things usually don't escalate that far.

I can't find a citation so maybe I am mixing things up. If I do I'll post it.