One of the Goals you can receive is to Complete X Trades. Trades are a good way to get rid of excess Cards and earn more Buds.
Trades are offered by Cheech at the bottom of your screen:
He asks for Cards you have in exchange for Buds:
It's up to you to decide if it's a worthwhile deal! If you don't think it's valuable, don't worry, Cheech will offer a Trade approximately every hour. Be sure to check out the deal even if you're just stopping in to open your Evidence Locker!
You can also get Trades in Events for Event Buds or Cards!