and will i get it at 80$ or below at used?
You have to be careful about which model you get. Some 7750s have DDR3 memory and you can find them NEW at around $80 and others have GDDR5 which provides about twice the RAM bandwidth and cost closer to $100 new.
With a GDDR5 HD7750, you can play most games with most eye candy enabled at 1920x1080p.
If you want to know how much these are worth used, eBay is your friend.