Flippa has no restrictions on which top-level domains can be sold on Flippa, outside of any barred by government regulations (like .edu or .gov).
ICANN has regulations that typically state a newly registered domain can't be transferred to another registrar within 60 days:
One way that this often works on Flippa is if you've recently registered that domain, the buyer would have to create an account with the same registrar in order to comply with the rules.
Since Flippa is itself not a registrar, we don't enforce those rules ourselves. You'd want to check with your own registrar for any particular restrictions.
Our rule that we do enforce is that you must be able to sell the property that you are offering for sale on Flippa. If your domain has been recently registered, you must clearly mention in your listing description that the domain is new and what a buyer must do according to your registrar's requirements in order to transfer it.