While it would be convenient and easy, I disagree. There would be a ton of #s up there and the auction house would be terrible. I would just put down 5,000 of a 5 stub card I can use for exchanges.... but probably have to wait until a few others' 5K orders went through. People would post up all over pricing everywhere.
Having to manually put in every order is a deterrent to spamming. Spamming still happens, but if everyone is able to put a quantity on everything, then it would get real bogged down.