1Google's number prompt
When anyone signs in to a Gmail account, Google shows a number on the sign-in screen and sends a prompt to the phone linked to that account. That phone shows three numbers, and the right one has to be tapped.
After the sale, these prompts have to come to our phone, not yours. Moving them to a new phone isn't instant.
2Everything we change
On day one we switch every way into the email over to us:
Our phone
Our number
Our email
New one
3Why we wait 7 days
Google's own help pages say a change to the recovery phone or email can take up to 7 days to take effect. During those 7 days, the old recovery phone and email can still be used to recover the account.
So for 7 days, the old owner could still tell Google "I've been hacked" and take the email back. After 7 days, that door closes for good.
That's why payment is sent after about 7–9 days, once everything is locked to us. It protects us, and every buyer we sell to after you.