Proof of ownership works for wallets the buyer controls: hardware wallets, software wallets, cold storage and multisig. It does not work for an exchange account, because the exchange holds the keys.
So when a buyer's crypto is on an exchange, tell them this: withdraw it to a wallet you control and verify that wallet. When it is time to fund, you can still send from wherever you like.
A buyer whose assets sit with an institutional custodian is in the same position: the custodian holds the keys. The preferred path is the same too, move the assets to a wallet the buyer controls and verify that. Where that is not possible, RealOpen has a manual path: a statement or letter from the custodian or exchange documenting the holdings and the buyer's authority over them, reviewed by RealOpen staff, who can approve a letter on that basis. It is slower and it is staff judgment, so start it early and do not promise the outcome.
A Bitcoin wallet usually holds its funds across many addresses. Every verified address counts, and the dashboard shows a Bitcoin holder how to get all of theirs counted.
Staked assets are handled in the same scan. Supported staking positions count toward the verified total and are disclosed on the letter as staked. The buyer does not have to unstake to be verified. They do need transferable funds by the time the deal is funded, and unstaking can take days.