I think the question he's asking is... how are you going to contact the original purchaser to verify that he's passed along the discount?

e.g., if I bought unit number 666, sold it to John Doe, and moved away (no longer valid e-mail, no longer valid address), is John Doe screwed because you can't get in touch with me?

Perhaps a better plan (or more clearly spelled-out plan) might be in order. (Tip: Include "proof-of-purchase" stubs with the new units when they go out, that will solve this problem and its ilk for future issues).