I was going to buy one from amazon and then turned out my local carrier store actually was selling it unlocked at a lower price than Amazon plus local 2-year warranty from Sony here in Australia.
Just be mindful when you purchase from any of them because not all Z2 comes with NC headphones. The Australian version certainly doesn't, I can tell you that. You might want to factor that in when you look at the prices.
In terms of build quality, I think you just need to play with the odds. Mine is 14w18 without any issue, yet some people with phones around the same batch are complaining about gaps and whatnot.
Someone was posting about a deal on eBay, Z2 for only US$599. I doubt you get warranty out of those resellers on Amazon anyway since those phones are not going to be the US domestic model. Why not go for the cheapest? (well, possibly cheapest)