I dunno what to tell you in regards to further actions, but I will say that when I converted to PnP on my Accord, Retro was known to have issues back then (08-09 I think). People would go out of their way to advise against purchasing from them, which is why I eventually chose Xenon Depot for my kit (highly regarded in most circles).
I cant imagine the Cherokee system can damage a ballast in any way, I would guess it was just a bad ballast from the start. Ultimately, you should use relays in the future to isolate the system from the ballasts...protects the vehicles electronics, and doesn't give the seller an out like they are claiming.
Granite V6 Trailhawk, Leather, Tow, Comfort/Convenience, 9 Speaker, Nav, Lifetime Maxcare
Adds: E46/ZKW-R/CBI HID Retrofit, RRO Super Sliders, JWS 6145 Fogs, Tint, LED Interior/License Plate Lights, Shark Fin Antenna, Black Muffler, Canvasback Cargo Liner, Weathertech Passenger Liners, Weathertech Window Deflectors