How to Trade Kuva Lich?

Only way to sell it is to first conver it then either you or the person your trading must have The Crimson Branch built in their dojo (also im pretty sure a Lich can only be traded once). Then just go to the Crimson Branch and from there trade the lich for whatever in exchange.

Can you trade a Lich if you have one active?

1 - you can't have an active Lich when you trade.

How do you trade KUVA Lich weapons?

No, kuva weapons can't be traded, only converted liches.