is cheapest, its chinese tho so you cant just buy using european payment methods, only wechat and alipay. Buff is the biggest skin marketplace out there
There is also skinbaron.de and skinport.com which both support western credit cards, on skinbaron you can also pay using paysafe and paypal but the only way to withdraw money is by bank transfer iirc
On Buff the items are sent directly from seller to buyer, on skinport and skinbaron the skins go through the site's bots
The above mentioned sites are all marketplaces, there are also sites where you can sell skins instantly but they pay way below market price and charge a huge markup when buying. There are sites like cs.money where it seems like you get a good deal when selling but you cant withdraw money directly
Try to stay away from gambling sites
