I've used AmmoSeek a few times when I was looking for some .223 and 9mm ammo. Overall, I found it pretty useful since it aggregates prices from a bunch of online retailers, which saved me a lot of time compared to searching each site individually. The prices are generally competitive, but I did notice that sometimes the listings would show as in stock when they weren't, so it pays to double-check the retailer's site. I've encountered a couple of sketchy sellers, so I recommend sticking to well-known companies with good reviews. Just be cautious and make sure you're buying from places that you've heard of, and you'll be fine.