Ive never heard of "NAGS" glass before. Per the add "This is a brand new NAGS brand (identical to OEM except the price) complete sliding back glass with frame - new!"
From some quick research nags stands for National Auto Glass. They certainly are not identical to OEM IMHO. I got a aftermarket front windsehild for my Tacoma and it looks like crap compared to stock glass. Yea I saved money, but in the corners it's not round and you can see where they heated it up then bent it at a sharp angle. This glass is much easier to make so that shouldn't be a concern.
Here is a pretty good write up explaining what you may get with aftermarket glass.
OEM vs. Aftermarket Windshield Glass - What's the Difference?
IMO who cares who makes the glass in the back so long as the rubber and plastic is good quality. Lord knows the Toyota OEM window latches can be broken by any crack head who wants to get into your truck so I think the OEM frames plastic and rubber sucks. In the front I'd stick with OEM.