Is it possible to add a new sort value 'New' and 'Used' as this would help buyers filter/find conditions of the items available more easily?

I think it's already there.

If you click on the "condition" column, the new (vs. sealed) -- which means a grading of 10/10 or 9/10 -- will be listed first.

Ex: https://www.lddb.com/laserdisc/shop/46/sort/status

Julien