I bet the "codes" are DL certificate codes issued from the redemption of MCR points before the Skymiles awards disappeared. In that case, they're probably valid and would probably result in the deposit of Skymiles into the buyer's account.

In the past, Delta has reportedly had good success in shutting down sales that involved the use of the seller's miles to purchase tickets for the buyer. Delta can (and does) reportedly compel eBay to reveal the seller's identity, and when the buyer's ticket is purchased from the seller's account, they have the identities of both parties and can shut them down.

It seems from what I've read that DL's ability to compel eBay to reveal the seller's identity comes from his misuse of their trademarks, such as "Delta" and "Skymiles." But if this is the basis, it would seem that they wouldn't be able to get the buyer's identity except by the fact that he's getting a ticket from the seller's account. After all, the buyer is not using Delta's trademarks at all.

So in the case of a cert code from MCR points, even if Delta gets the seller's identity, how do they identify the buyer? EBay would not know the coupon code, so they would not be able to reveal it. And the buyer would not be misusing DL's trademarks, so maybe eBay would not have to reveal his identity. Maybe.

What I'm saying is that I think in this case, the OP might actually have a better chance of not getting caught than in the more conventional case.

Then again, even if I knew all of the above to be solid fact, I wouldn't do it because it violates terms I've agreed to by joining Delta's Skymiles program. And since the above involves a lot of hearsay and assumptions, it is a risk I wouldn't take myself, even if I didn't care about playing by Delta's rules.