I e-mailed the seller, and he's sent a couple of responses already. Seems genuinely puzzled that it would suddenly show up as stolen 4 months after activation. The listing was gone from ebay, but I still had an ebay message from him with the receiver (R00) number, so I was able to document that this was in fact the receiver I bought from him. If he's a scammer, he's a bold one, based both on the response, and the fact that I have his name and address from the shipping box.
I also sent an e-mail to the "ceo" address last night. I've always gotten a response to prior e-mails to them, so I hope to hear back today. Do they keep records of CSR chats with my account? If so, they could probably find the one where they approved it.
There was a prior thread where the guy who runs dishstore.net said that if the price is significantly lower than his, it's likely stolen, as it would be below wholesale cost. I think the price I paid was $30-40 less than his at the time (although not now!), so perhaps this was the case.
Anyway, my current thinking is that if Dish won't change their mind on it, I'll try to just lease a receiver to replace it. Since there's a $7/month fee either way, it's a wash going forward.
If I can get the seller to refund my money, all the better. But if not, I'll be in no worse shape than if my purchased receiver was still working - out the $$ I paid for it, and still paying $7/month in fees. Or am I missing something here?