There are many ways to find deals on CS items but clicking a bad link is very easy.
I use csgoskins.gg for accurate pricing and multiple trustworthy marketplaces. 
(Note that market places like Buff 163 are not availlable outside of China and can vary in pricing)
 - csgoskins.gg

For buying and selling anything over ~$50 USD I'd HIGHLY recommend going through CS Float as the fees (2%) and prices are some of the best on the market

 - CS Float

For 1 to 1 trading anything under ~$50 USD (this is were fees dont matter nearly as much) Id recommend Dmarket as it has some of the most competitive rates out of the trading sites. (I do not recommend trading higher ticket items unless it is globally the lowest price)

 - Dmarket