50U7A is identical in features to 55 & 65 U7A models, the only difference being that bigger screen size has more dimming zones which is the case in every LED TV with dimmable zones.
Regarding buying used LED TVs, well it depends on the usage of the first owner. LED TVs have a Backlight or Edge light which has a usage life in hours like any other light bulb and it's luminance reduces with usage. So, it depends, on how much it was used, if it was a display showroom TV which was running on showcase mode at full blast for hours everyday for 1.5 years, I wouldn't buy it but if it was in a household with moderate usage and it seems fine to your eye on tests, then it might be worthwhile if the price is right. What is the asking price if I may ask.
So run some tests, a good basic test is a white background, then light grey then dark grey background to check for dirty screen effect, lightbleed, irregular dimming, dead pixels and effective nits/peak brightness. The older the LED TV gets, the nits drop and the screens also develop a dirty screen effect. These TVs have only two major components, Backlight and Motherboard, so check all inputs as well. Anyways, best of luck.
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