Large ammo purchases were done by James Holmes prior to the shooting.
What do you consider large and what exactly would you do with the information that someone purchased a large amount of ammo? Do you set a limit on the number of rounds a person can buy per month, with each store performing a check before selling to you? Do you just track it and require a mandatory police visit for anyone who buys over the monthly limit? Do you jail people for purchasing over the limit? Do you also tracking reloading supplies to limit the number of rounds someone can reload? What's to stop someone from buying up to the limit each month and hording? Should police enter your house to count your available supply of rounds once a year?
Do you think a crazed person who thinks that going on a rampage is somehow the acceptable and correct thing to do cares whether they have 300 rounds or 5000 and that this will somehow stop them from shooting?
Take the case of Holmes. Yes, he bought several thousand rounds, but he shot maybe 100. A majority of his shots came from a Remington 870 and a Glock 22. These shells and bullets typically come in quantities of 25, 50, or 100. I doubt he reloaded the 870, given that it doesn't have a detachable magazine. So 6+1 12 gauge shells, ~30 .223 rounds, and ~60 .40 rounds. So he could have bought a case of 12 gauge shells, 2 boxes of .40 rounds, and a box of .223 rounds and accomplished the same thing. Do any of these numbers sound like they're over whatever arbitrary limits you think there should be? Do you think that Holmes wouldn't have gone of a shooting spree if he only had 100 rounds?