Opskins or CSGOlounge (maybe some people here even want to buy something...) and if he has something special people will give more money (look at the float on opskins to gain extra money) or ( for csgolounge the float and or layout (and rarity) of the skin (blue gems etc.)
heck maybe even try reddit if you he has something special!
(BTW be very careful my friend made 1000 dollars from 50 dollar and he betted everything away so know when to stop and cash in! )