~ people do buy and sell accounts with private keys attached. That's just a fact.
An
inaccurate way to check this would be if the account owner dares to keep a substantial amount of money on the staked address for a sufficient amount of time. Say $500 for at least 2 weeks.
If the account is bought including private keys, chances are the seller takes $500.
In that case,account would be sold with the private keys and the money attached.That amount can be easily paid by the account buyer to the sellers another bitcoin address.This increases the chances of scam but can be avoided if the trade involves an escrow.