Looks like I am one of those who fell for the clickbait 
This finally makes sense to me as I was wondering why anyone would want to buy a wallet they can't use other than to sign messages or stupidly keep funds the original owner can access.
I don't know if proving it came from capital gains will require this kind of proof though, most bankers will still ask for screenshots of transactions from your email or exchange in those dates, not signing. I say this from exchange requests asking to show where it came from.
For the buyers, I always assume it has to do with taxes. "Proving" you owned Bitcoins a long time ago, and your current riches really came from capital gains and not crime. Or just bragging rights, but it seems like cheating if you bought the empty address.
That part about taxes is not very logical to me[/quote]This finally makes sense to me as I was wondering why anyone would want to buy a wallet they can't use other than to sign messages or stupidly keep funds the original owner can access.
I don't know if proving it came from capital gains will require this kind of proof though, most bankers will still ask for screenshots of transactions from your email or exchange in those dates, not signing. I say this from exchange requests asking to show where it came from.