i dont touch the TLC ones personally especially used. You can get a brand new sandisk extreme pro for 220 on amazon if you price watch. you can get cheaper ones of different brands like crucial that are also MLC
I've got no problem with the 850 EVO's.. Stay away from the 840 EVO's though.
That being said, I also wouldn't go the used route. No telling why that thing is so cheap. See if you can get him to show you the Samsung Magician info for the drive before buying?
I would stay away from 840 and 850 (they are newer and there are not enought feedback data yet, but it may be as bad as 840 I guess).. and the second thing..will you really find usage for 500Gb SSD?