I'd be careful about selling a "loaded up system." Nintendo has sued some people for doing that on ebay. plus, it might be removed at some point if ebay finds it.
the nes mini is easy enough to hack. all games for one region will fit on internal flash, but you can add more with an otg adapter and a flash drive. I personally just put all nes games from one region on my system, since I want games for that system to be on that system and only that system. it can't really do anything more than a pc emulator can; in fact, some games don't work with the original emulator. you can change the emulator to retroarch though. thus far, I've not seen a game that didn't work.
I guess it might be more aesthetically pleasing. you can use google images to download artwork for the games. I think hakchi will even automatically resize it for you.
anyway, I think the selling point is that it uses hdmi and that it's compact. the cables to the controllers are awfully short, so I just bought some 8bitdo, wireless controllers. those even include a reset button on the controller. I wouldn't buy an nes mini thinking it will do anything better than a pc emulator. I think it's just convenience.