I currently have the VIP622, but an thinking about buying another box (Vip211) from a private seller. If I do, do I need to let DISH know so that they can allow my box to see the signal?
Yes you must have it added to the account and authorized before it will receive programming. You would be responsible for any balances on it and it would add $5 a month to your bill.
As a precaution, you should get the receiver numbers from the seller before purchasing. Then, you can call Dish and make sure the receiver can be activated, and has no outstanding balanaces or is not under lease.