Stock Tab S 10.5 won't turn on, then stuck on logo screen

I just received a used Tab S 10.5 from a reputable Ebay supplier. It didn't turn on initially so I plugged in the charger and it showed a solid gray battery icon with a solid white lightning bolt. I assume that means it's fully charged. Still wouldn't turn on with the power button so I tried holding the power and volume buttons together. I'd get a short vibration after 6 or 7 seconds and the battery icon would go off but it wouldn't start. I finally got it to start (not sure what I did differently) but now it has been stuck on the Samsung Tab S opening screen for several minutes. I tried turning it off with the power button but holding it down for about 10 seconds had no effect so I let it go and the logo screen stayed on.

Before I contact the supplier for a refund or exchange I would like to know a few things. Are the Samsung Galaxy Tabs finicky about turning on? Any known issues with power buttons? What's the proper procedure with the power button alone doesn't work? I've seen both power + volume down and power + volume up.