If you wish to sell CSGO skins you can do it at Steam Community Market, third-party marketplace of your choice, and at Skinwallet. Steam Community Market's biggest advantage is that it is practically impossible to get scammed. The prices are also quite high.
Demos downloaded this way lie here: & \\Steam\\steamapps\\common\\Counter-Strike Global Offensive\\csgo. We also recommend to place demos downloaded from other websites into this folder. It will make it easier to launch them later.