Ebay Sell?

Agree with DD - no one really wants the stock system - after all they are all going to be doing what you've done and fitting a better system. You might get someone who wants the standard pipes but I doubt it. Best to keep them and put the bike back to original when you sell it. You'll get more and then be able to put the new pipes on ebay - which will sell!
I had TORS fitted to my Bonneville at the dealer and my stock pipes are still there in a container in the back yard!