It depends on the domain registrar you're using, but many popular registrars don't allow *external* transfers of domain names until 60 days has passed from either the first registration or the latest transfer.
Mind you though that I said external. All major registrars allow domain transfers within the same registrar without any limitations. So if your domain name is with say GoDaddy and you want to sell it earlier than 60 days from when you registered it, the buyer will need to have an account with GoDaddy as well.
With that said, it's always good to have the buyer register an account with your registrar, regardless of whether there are limitations in place or not. That's because transfers within the same registrar are typically instantaneous, whereas transfers to a different registrar take up to a week (2 weeks in some cases) so it adds a fair bit to the total time it takes to get the site transferred over.