My barber says I need a haircut (he's right at the moment, but that's not the point)
The local ammo shop says I shouldn't order online to get ammo in bulk, the feds might harass me about it
Seriously, without a substantiated or first-person account, this is more likely to be a myth generated to either:
1 - sell more ammo over the counter (seriously, the SHOP doesn't order ammo in bulk?)
2 - make the BATFEIEIO look more intimidating
3 - pass the boring hours in a gun shop
There is no way to tell what's in a box, other than a single round of ammunition requiring an ORM-D sticker/label. The CMP sells pallets of ammo to regular folks, match shooters order mass quantities, clubs get together and make group buys ... hell, I've split an order with someone, and I'm just some random guy living a stone's throw from the border with Canadia.
If F-troop comes to your door asking about the ammo, ask to see a warrant and get names from all the clowns trying to intimidate you, then politely close the door.
But they won't, the manpower isn't there and the organization it would take to gather that info AND act on it doesn't exist, these are the dregs of the fedgov we're talking about, they can barely sell guns to criminals on time or under budget, they aren't investigating your bulk ammo purchase, and if they were they'd screw it up somehow.
And for some related pictures:
A related link:
http://www.odcmp.com/Sales/ammo.htm
Look at the CMP sale site there
They recently sold out of AP 30.'06 ammo
They sell .223 by the case (I got a discount diring the national matches, even! 30carbine, too!)
They're selling "Aguila .22LR Caliber Target - 1200 fps - 5000 Round Case" for $305 +S&H (I know a guy who orders a couple of these for a youth rifle team every year)