Trade system is flawed (even if the cost is changed), here's a suggestion
The current incarnation of the trading system does not encourage trading. Offering a card up without being able to request a specific card back makes the assumption that all cards of a given rarity are equally desirable.
To use this trade feature, I need to be able to communicate with friends outside of the app, to organize what cards both parties want to trade, since there is no way to see another person's collection or request a card to trade for. This pushes players off platform, to places like discord or reddit that can be unsafe for younger users.
The app already has a feature to show off your collection. Binders and Collections are effective tools for showcasing cards. Thinking to the real world trading card hobby, passionate collectors maintain a "trade binder", with duplicate cards or valuable cards they're uninterested in, and offer this collection to other collectors to make offers for their cards.
In TCG Pocket, a trade binder would make trading fun - browsing through your friends' trade binders and seeing the card you need would be exhilarating. A trade binder would be competitive - knowing that a desirable card is visible to many people would encourage fair offers. Most importantly, a trade binder would be effective - people would actually be able to use this feature to complete their collections or build out a new deck.