Before releasing the account information to the buyer, change the email first
What steps need to be done to change the e-mail address on the Warframe Account?
1). Warframe Update Email Address FAQ
2). Create A New Email Address (Example: Yahoo, Gmail, or Hotmail)
3). After creating a new e-mail address, Submit A Support Ticket with Warframe. Provide in the details that your old e-mail address was being used by your previous company you worked for and you would like to update the e-mail address to this new e-mail address.
4). Once Warframe confirms the e-mail address change has been completed, please complete this form submission and provide the new e-mail address below. We will not be able to complete your account submission unless the e-mail address has been changed.
The alternate option is just change it to the buyers e-mail address directly which the buyer may prefer versus creating a new e-mail. In order to do this though, the buyer will need to provide his e-mail address to the seller.
When submitting a ticket to Warframe, you can say something like I am transferring the email over to the company I previously worked for and need to change my email since I will no longer have access to it soon. Don't mention you're selling this account.
After that, Warframe will ask some question and then you'll be all set with changing your email over to a new email.
The recommendation here is don't transfer the account information to the buyer until the email is changed. Either create a blank gmail yourself or get the email address from the buyer. The reason for this is Warframe will know you're selling the account if they see a completely different IP in another country accessing the account at the very same moment you're asking for an email change.
Once the email has been changed, recommendation is don't login to the Warframe Account. Just transfer it over to the buyer.