Simply type !buy "Number" Mann Co. Supply Crate Key if you want to buy keys,
and !sell "Number" Mann Co. Supply Crate Key if you want to sell keys.

For example:

!buy 5 Mann Co. Supply Crate Keys
!sell 5 Mann Co. Supply Crate Keys

You can also buy or sell anything through our website: https://quicksell.store/

Good luck with your trading.