I sold a domain to a gentleman who wanted it badly (hi Kyle), and had been using it for much of my personal business.
He just forwarded the email from my requested addresses over to my new email address. No big deal on his end -- any mail tosser worth its salt makes this easy.
It was completely informal, and it gave me time to transition to a more permanent (and less contentious) home.
Eventually, I dropped the "new" email address that Kyle was forwarding to, and haven't really missed it.
Just sell the thing (if its worth anything), ask for your email to be forwarded (and make sure it actually is being forwarded -- not everyone is as easy to work with and so adept as Kyle) and leisurely begin changing your email address at the places where it is important to you to do so.
It's a minor pain, but the worst that can happen is that your (presumably honest) buyer drops the ball and the forwarding never works right all, but then your Really Important Personal Stuff is still relatively easy to transfer.
People switch ISPs all the time, and a lot of the time change their email because of it. Banks / creditors / utilities are used to it, and have systems in place to deal with it. A bit of a PITA, but, seriously: It ain't so bad.
And the best case is that the old address(es) work fine for years to come. Your job is to not rely on that continuity, and to just get the hell off of the domain you've sold as soon as it convenes you.
Lawyers and contracts and explicit agreement are useful for times when you don't trust someone.
And if you don't trust your buyer, don't sell to them. Nobody's forcing you to, and a domain name and hosting is very cheap these days (especially for something that you no longer care about) compared to a lawyer or even the time discussing the matter with one.