Selling a Wordpress site..

give them an FTP account with restricted access...

In order to get a restricted user level, the domain needs to be a subdomain.

The client needs to realize that the designer really needs access to cpanel to do the job for them.

When I have needed access, I just tell them to change the password after I am done.

I've installed hundreds of sites for clients and if they don't want to give me cpanel access (very rare), I give them a link to the files and instructions to do it themself.

If you're not familiar with what FTP is, it's going to be a big headache to try and move the site and mistakes are inevitable. Not what you want when you just want to transfer it and get the money.

If it's a big move, I would hire someone local on Craigslist to just meet up with you and handle the site transfer for a small fee.

The question is pretty general but mostly you should be working with a designer you can trust.
That said, if you're getting a new site up that includes setting up email, domain, database etc, then yes he/she needs access to get those items done.
Otherwise ftp access to a specific folder/directory is all its needed.

if you have only one domain/site setup in that cPanel, then go ahead and give them access with a username/password that you're going to change when completed. any damage can be wiped out if necessary.

For ftp access just create an ftp account for that sole purpose and delete when done.

For a website repair or admin access in WP, again create an admin account that you will delete when done, but not before backing up and downloading your whole site to your computer or similar including the database.

If you're talking about access to a cPanel that has multiple accounts you don't want available or compromise, then you or a trusted friend should be able to do what's necessary to give the designer everything they need in order to limit them to an ftp account only or admin in WP.

There's a number of ways to do these things and you can always ask your host to what they prefer and they may able to help setup or do it for you, depending on your host of course.