It's not something the forum itself should be doing to be honest. I was thinking it would be something the user himself would be able to do. But I just don't have any good idea on how to display them.
That way there is no issue about scammers, and anyone who wants to talk to the user on the other platform can just verify with him/her that they have the account ownership.
So if I get your point correctly I think you are suggesting an extra information displayed on users profile something like Exchange name (which would be specified by the user) and your trust rating on that exchange (also specified by the user) ? So probably other users would have to toggle to your profile on that exchange to verify right?
Still users often make offers on the service board and if it relates to these kind of transactions they usually leave a trust feedback most times including the means of the transaction. Anyways let's see what other users here think about the idea though.