I can see how this will be easier for many casual users to remember, but personally I’ve got so many eBay accounts I’ve lost track of which email address is associated with which account. However for consumers, especially if they don’t have a memorable eBay ID, their email address will often be easier to remember.
It’s a sensible change to give the option of email or User ID, in fact thinking about it I don’t know why eBay didn’t enable this years ago.
If you are in the trial you may find that email sign in sometimes fails. Don’t worry you’ll still be able to sign in as normal with your eBay user name as the test may be turned on and off intermittently.