Nice of Sennheiser to come through for you on that one!
I'm of the opinion the majority of breakers don't really give a toss about the parts they're stripping off vehicles and selling. Certainly in the case of parts that can and do fail, I don't believe and of them them 'carefully test before removal' at all. Strip it off the car, list for a low price, offer 30 days warranty. Hope for the best.
There are certain things you just can't test without either specific diagnostic kit or the right conditions, and I'm pretty certain a breaker isn't investing in the kit for all the weird and wonderful things fitted to different cars on the off chance they'll come across some to sell...
Great if it plays out, a load of hassle for the buyer to fit an awkward as hell part only to find it doesn't work. At least with eBay/PayPal, for the buyer it will usually play out in the end. For sellers though, like in your case, it can be a nightmare. I've sold stuff on eBay before, but I can't be bothered anymore, too much potential hassle.
Southampton with a Nanocom for Thor/GEMS P38s
1996 4.6 HSE, then became a 4.0, now cubed.
1997-8 4.6 HSE Motronic/Wabco prototype vehicle. Now M57 powered. Still auto.