The Trading feature is unlocked when you reach Garage Level 3. Both Blueprints (BP) and Import Parts can be traded for Trade Coins in the Inventory (BP only if the car is unlocked already). Blueprints are auto-traded when the car is maxed out. Import Parts are auto-traded when they exceed their inventory limit.
There are special slots in the black market where you can get Blueprints or Import Parts by paying Trade Coins.