This probably works good if you sell lots of receivers. Potential buyers can see sellers feedback for selling receivers and their customers sucessfully activating them. It's a little different when you just want to pass one or two on to upgrade.
I have sold two on eBay and I passed the R & S numbers on to the buyers before the sale and got a pretty good price on both. Within the past couple of years a 2700 went for $30 and more recently a 4900 for $50.
I am soon going to "trade up" an 811 and I expect that to go for over $100. Then I will get another ViP211 for about $200 or maybe a ViP622 for $350.
If you are patient and watch closely, you get the best deals. When I buy, I want a good price and I want to check the numbers with Dish. If the seller does not share the number with me, I will not buy.
I have had no problems when I share the numbers on the receivers I have sold.
What could someone do with those numbers anyway?