I have bought many coins on eBay and pretty much follow the same guidelines. High feedback and longevity are key. I also have acquired a long list of favorite sellers who I usually check them before making other purchases. EBay can be a nightmare if you're not careful but if you use some common sense you can find some great deals.
Unless they state the variety on the holder and you can confirm it be careful. There are a lot of slabbed Machins out there that aren't Machins.