Hz is important but there are other factors that are important too... i know that the LG has better black levels and colors than the Toshiba and RCA which makes the picture better....and it is a acceptable trade off for such a cheap TV...
$500 does not give you a good "New" 50" tv... especially for tech geeks, gamers and people that loves movies....
a $500 tv is for a typical soccer mom that only watches soap operas... as that is about the only thing it is good for ;p
My Mother (that don't know a single thing about tech) bought a random 40" samsung TV on sale a few years back to have in her living room and it had 50hz and terrible colors and black levels.... she uses it for daily use as she only watches boring shows on it.... but it lags like hell if i try to watch a action movie on it.....
At that time i had a 6 year old $1500 LG 42SL80 LCD that had 200hz and average colors... and i could see a huge difference between my worn out old Lcd TV and my moms brand new crappy $500 Led TV....
Earlier this year my little sister wanted a new TV to play video games on (as she had a crappy 30" Samsung LCD that my mom got for her several years back) so i gave her my old TV and bought a nice used Plasma... and it is serving me well..... i love the 600Hz and life like colors... it is as i can climb into the TV lol... i suppose it is like this for 4000$ LED's too as i have seen some awesome Sharp TV's...
Anyway i'm getting off track here...
I assume that you are going for a Used Led right?...
There is nothing much to worry about when buying a Led/Lcd as it is not as fragile as a Plasma....
TV's usually comes with a 2 year warrenty or more... when you are buying used make sure that the seller have the Recept from the shop.......it is usually not a big worry as most people buy stuff online now so the receipt should be on his account....
Watch out for Third party TV brands... only buy TV's from the major corporations...
Make sure you that there is no Buzzing coming from the back of the TV... as that may be a problem if you are buying old tv's......that problem usually comes when the TV have become overheated and the power regulators / adapters get worn out....i dont think it is a problem with led but it was with lcd and plasma tho
Make sure there are no scratches on the screen and that the TV comes with the remote and manuals....
and make sure that the TV comes in its original Box or it may be troublesome to move it if it is 40"+...
Oh yeah and always make sure that you get to try it before paying.