The value decreased when you disclosed the confidential information.
Which itself has nothing to do with the 'contract'. What is the 'confidential information' in your eyes ?
The only thing which is 'confidential' is the PM i received. And this PM itself did not decrease the value.
The fact that i called him out for doing shady business is what decreased the value. And ONLY if you really want to call it like that.
Because the accounts had no real value. They were sold for a price. That's it. But the real value was close to zero.. it is just some shitty account which is being traded. No value behind it.
By the way.. i don't have a problem with tagging account sellers and their accounts. Even if they 'lose value'.
I know that sounds harsh to someone who owns multiple accounts.. but it is the truth.
It is the exposing the information he gave you that caused the damages. In conjunction with your not following through on what you said you would do causes the flag the person could create to be valid.
If you want to go around damaging the value of what other people are selling, I think that is kinda sleazy, but go ahead and do that, just don't fail to follow through on your obligations in the process.
The offer does not expose a person to liability. In order for an offer to obligate the person making the offer, it will need to be accepted by the other party prior to the offer being withdrawn, or expiring.
There were so much things missing regarding the 'trade' that it wouldn't even be called 'similar to a contract' in my country..
You can learn about contracts
here.