Just an update on this: I was able to track down the reseller after all. It looks like they just rebranded their eBay account to senkogames and I was able to get in contact with them through the new account name. Senkogames were extremely helpful and ended up allowing me to return the faulty...
My EZFlash Jr has randomly stopped working after being in storage for 2-3 months. It was in dry, waterproof plastic container with the rest of my GB carts and hasn't been knocked about.
When it starts up, it shows a black bar + (R) instead of Nintendo logo on boot.
I have tried the following...