Delivery address handling
The address is the only piece of the transaction that identifies a place. Everything about how it is sent and stored follows from that.
Encrypt it, every time
Anything typed into a message box sits on the platform in readable form unless it was encrypted first. Encrypting to the seller public key means the platform stores something it cannot read and the seller decrypts it on their own machine. This is the one message in a transaction where it genuinely matters.
Take the key from the profile
Not from a message, and not from anywhere that arrived alongside a request for the address. A key supplied by whoever is asking protects you from nobody, which is the same failure that makes an unverified signature worthless.
Format it plainly
Exactly as the postal service in that country expects, with nothing extra. Notes, instructions and explanations add information without adding accuracy. A name that receives post at that address is what is required, and inventing one that does not creates a problem on delivery rather than solving anything.
What a sensible vendor does with it
Decrypts it, writes the label, destroys the plaintext. A seller who keeps a readable list of addresses is holding a liability for everybody on it, and one who says so openly is telling you something useful about how they operate.
Do not send it twice
Resending an address because the first message went unanswered doubles the number of copies in existence. If there is genuine doubt, ask whether it was received rather than sending it again.
Never send it outside the platform
A request to send the address by another channel removes the message record and the arbitration route while handling the most sensitive item in the transaction. There is no version of this request that improves anything for you.
Why this is worth more care than the payment
A payment is a number that moves. An address is a place with somebody in it, and it is the only element in the transaction that cannot be rotated, replaced or written off. Treating it as the most sensitive item rather than a routine field is the correct calibration.
Check it before sending
A transposed digit or a missing line produces a parcel that fails delivery and enters a returns process, which is a far worse outcome than a slow one. Read it twice before encrypting, because correcting it afterwards means sending a second copy.