The various configurations come & go a lot on Amazon, and the prices can fluctuate from week-to-week. If there's a certain model you're eyeing and they either don't have it or the price seems a little high, you can watch & wait and it'll often show up or hit a better price point.
In terms of retailer credentials, what you really want when purchasing an Asus is the best return policy you can find. If there's a hardware defect, you don't want to mess with repairs, RMA, etc...you just want to send it back. It's hard to beat Amazon, as they'll take any unit back in the first 30 days for any reason, or for no reason. (If you happen to have Prime shipping, they even cover the return shipping fee.) Serious problems that pop up after 30 days will be serviced by Asus anyway, so the retailer is pretty much out of the picture after their return window closes.
(Note: Units purchased through certain retailers...like BB, Staples, Costco....don't qualify for the Asus "extra" warranty features, whether that matters to you.)