What if I wanted to trade Borderlands to a retail store since I'm 100% done with the game but what should I do with all the DLC packs I have bought for the game?
Put them on a flash drive and trade it in.
Seriously, what do you think you can do with DLC?
Delete them off the harddrive?
What else will you do with them if you don't have the game needed for them?
Pretty much this...
But just in case you also ever get the Game back again and want to play them again, simply DL again. They will always be on your Profile.