DSR will only apply if they are a business seller based in the UK. Also as it is over a month, DSR won't apply.
Sale of goods legislation would be relevant for UK purchases. On the international side, no idea and probably not cost-effective to explore
As they held themselves out as UK and are not - ie you were effectively conned out of your UK statutory rights, I suspect ebay will see that as probably grounds to set aside the transaction and reimburse you. That is the way I would go