As far as WalMart goes, I think I've figured out two major problems with BrickSeek. The following is based on several failed attempts and one successful attempt to purchase discount ammo today.
I snooped over several WalMart employee's shoulders as they used their little inventory computer-phone-thingy. When they ran an SKU or UPC, four numbers popped up:
"Inventory" and "Checked Out" are the important ones. "Checked Out" means the item was rung up somewhere and its quantity was discounted from the "Inventory" pile. 10 shotguns go into "Inventory" when loaded to the shelf from the pallet, 8 shotguns are eventually sold and go to "Checked Out," 2 shotguns remain with clearance prices. When "Inventory" equals "Checked Out," there is nothing left in stock for that item.
HOWEVER, the problem is BrickSeek only measures "Inventory." So us r/gundeals guys see 10 shotguns in "Inventory" still on BrickSeek. So we call the store saying there is 10 in stock. Not true, says the clerk over the phone. Additionally, BrickSeek delays its deals to non-members, so members get their first. For some reason, clearance items do not go into "Checked Out" and remain in "Inventory" or perhaps to a separate bin we're not aware of.
Not sure if this is all true but I think this is mostly accurate.