I need help

Let's say I open a chest worth 600 gems, then I get something worth 399 gems. Can I sell back the thing I got for gems?

Yup if you contact to author

So I can't sell it back immediately?

Yes, you can sell it back immediately, but you'll only get half of what it's worth, so if it's 399 gems, you'll only get 199 for your random steam key that you'll never use. I learned my lesson with this the hard way. If only I had known there is like a 1% chance to actually win anything decent from a chest. I'd be interested to know if anyone has gotten a decent game from any chest.