So, my unsolicited attempted buyout arrived, FedEx was able to find/google/call me even though that thing was sent to a nonexisting street adress (yes, in fact: "650 Castro St", located here in Berlin/Germany).

As From-address they listed:

Refunds@Hashfast.com
100 Bush Street
San Francisco, CA 94104
USA

(So this seems to be "the" confirmed address of them, btw.)