Keep the cards. There of no use to the buyer. They will need new cards unless they already have an account with active cards.
D* will activate used equipment, just not previously used cards from another acct. As recently as 2 weeks ago, I activated a used SD box with one of my existing cards after my D10 died. Later this week, I'm upgrading my HD box and plan to use my existing card in the refurbished receiver. You have to ask for a resolution specialist if the first level of CSR says they can't do it.