Player Trading
There is currently no player-to-player market or trading system.
Not currently available
As of this writing, Bubble has no way to trade, gift, or sell items directly to another player. There is no /market or /trade command. The only way to convert a fish into coins is /sell, which always sells to the game itself at that species' current dynamic price (see the Economy & Museum article), never to another player.
Staying safe
Because there's no built-in trading system, treat ANY offer to "trade" items or currency with another player as a scam. There's no bot-mediated way for it to happen safely, so it's almost certainly someone trying to get you to hand something over first outside the game.