I agree with the above. Buying from ebay is a bad idea. I would guess the 'codes' are actually mycokerewards points and coke has 'run out' of most of the delta skymiles awards so not much use for that purpose I am afraid.

In addition to buying the miles from Delta you could consider;

Get a Delta Credit card (if you don't already have one) - the gold card for instance will give you 15,000 - 17,500 miles with no fee for the first year (so you can cancel within the first year and pay nothing for your miles. See: http://www.rewardsdb.com/finance/US_Airline_Rail.html

You can earn up to 2000 skymiles per year from erewards.com for completing online surveys.

500 miles for getting a free Ameriprise quote (see Delta.com -> Ways to get SkyMiles -> Real Estate & Finance)

2000+ miles for opening a SHarebuilder account (same section as above)

And then of course you can shop online with any of Delta's partners (see RewardsDB.com for an idea of the range of shopping partners.