The same happened to me months ago, I'm sad to say this but there's nothing you can do about it. Valve support is really trash and I doubt they will deal with it, the max you can get is a ban on the guy account, that's all.
I lost 6 CSGO skins worth around 500 usd. I basically got scammed on a fake gambling site, lost the skins, reported and didn't get anything in exchange, not even a ban on the guy.
Big oof. That sucks ass.
I'm the Event Squad Admin. I am also an ex-Clan Squad member. Have any questions about clans or otherwise? PM me.