You push the domain to the new owner's account and you contact (and change the name) your hosting Co...
I am not sure what 'flipping websites' is but if it means selling your websites then what you normally do is to push the domain name to your customer and as for the websites, you could either provide your customer the access to the site admin so they can transfer the site over to their own server or you can zip the site up and email that to them. They will usually be responsible for the transfer of the site.
Flipping website is like flipping house where you buy a potential website, develop it and the sell it for profit.
Let the buyer know where the domain is hosted so he/she can open an account there for a free push. After the domain is pushed you send the backup of the site plus any databases. CPANEL is easiest for "flipping" Popular registrars are: GoDaddy, NameCheap, eNom, Moniker, etc. - you get the picture -