Can I sell a .eu domain to a registrant outside the European Union?

The key point is that once you've sold the domain, you would need to transfer "the ownership" or more accurately "the registration" to the new owner, who would need to be an EU resident or organisation.

When you take ownership of a domain name, regardless of who you bought it from, you then need to register the ownership with ICANN or an accredited registrar (in this case EURid).

See the "Transfer your domain name" page - as this trade will need to be mediated by an accredited .eu registrar, you'll need to find one who is willing to neglect to check that the new registrant is an EU organisation/resident (presumably opening themselves up to losing their accreditation).