usb display adapters

I have an android tablet with a large 14-inch display (Virgin Media Telly Tablet) that I would like to re-purpose as a computer display. The thing is I need it to work as seamlessly as possible, ideally just like a TV, as I am intending to use it for a relative who has little computer knowledge...

I recently built my gaming pc, but sold the monitor to upgrade to a new one. Before I sold it, I used to gamestream to my laptop using Moonlight and nVidia's GAMESTREAM even with the monitor turned off. But now that there's no monitor connected, the GPU won't turn on, so I can't run games. Is...