If you buy at Amazon, you can check that you are buying FROM Amazon, not just VIA Amazon. If it says: Sold by Amazon, Ships from Amazon, then it's reputable vendor (in my experience!). If it's only fulfilled by Amazon or just sold through their website, then things may vary.
Amazon is good at marking units as refurbed. Of course, there are 3rd party sellers on Amazon as well, but if they're caught misrepresenting the product they get banned pretty fast. I would have no hesitation buying from Amazon, but make sure they state the specific model number being offered. There are multiple versions of the Stick, Express, Premiere and Ultra, and all have slightly different hardware. If you want the new Ultra that supports Dolby Vision, you have to make sure you get the model 4800.
Dan Ultra (4640), Ultra (4670), Ultra (4800), Premiere (3920), Insignia 720p Roku TV, Sharp 4K Roku TV, Nvidia Shield, Windows 10 Pro x64 running Serviio and Plex on a wired Gigabit network.