It'd be a download code.

You can only sell real life items in the RLT section within the Donor Forum. If you do not have DF access refer here: http://forums.d2jsp.org/info.php?p=30

If you do have access the best thing to do would be to ask Kevin, he runs and mediates that section. I'm not entirely sure on the rules on selling digital download codes so best thing would be to ask.

you probably got it from humble bundle
so you cant sell those codes
and every1 that wants bf3 already has it by now so just give it away