Wallet B was a lost, even though it was more than wallet A, how you keep track of that can be difficult, My exchange however does list all my transactions, figure out what transactions were Gaines, and what was a lost will be heard. I would guess, this is where people will fudge a bit, because it would be hard to prove which is which. Reality is, smaller transaction will not be examined, at least for a while, transactions above I think $9k will likely have to be reported by the exchanges.
However, I think would will come of this, is countries setting up untraceable/ Tax free exchanges. and a continued, attempt to block them.