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People change keys for all sorts of reasons. Seeing that on the blockchain means very little.

I agree ... a more complete observation is that a change of keys does not necessarily imply a change of ownership, but that the converse does (or one would hope).

Well if they actually change keys yes. But if they just sell the account for cash on ebay and hand over the username and password or the wallet file directly then no.

Now you and I might not be stupid enough to do that. But vanity bitcoin addresses are traded like pokemon.