I know that I would not submit nearly as many cards if the fee's to "list" were double. What's great about COMC is being able to buy many cards and having them shipped to you for a reasonable price which is a service that Ebay doesn't provide.
I know for myself, I love buying more cards knowing that I am only paying an extra quarter to have them shipped to me even if I buy from many different sellers. I know I've noticed, COMC shipping prices are cheaper than most Ebay sellers.
PS Forget Strasburg... Jordan Zimmermann is the Next Big thing in DC in 2009 and Beyond!