I've used AmmoSeek.com a few times while hunting for 9mm and .223 ammo, and I found it pretty straightforward. It really does aggregate prices from a bunch of sellers, which helped me score some deals when local stores were running low. That said, I always make sure to double-check the seller's reputation, especially if the prices seem too good to be true. I ran into one seller with a pretty sketchy rating once, and I backed out after reading some reviews. Most listings are accurate though, but I recommend checking shipping costs since they can vary a lot. Overall, it's a handy tool, just proceed with a bit of caution like you would with any online purchase.
