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 Nanospiders.

Trades are offered by Frances, Rory or Janice at the bottom of the map screen. They only appear every couple of hours and only when you have enough copies of a card for an eligible trade. Which quantities of which types of card depends on where you are in the game, as the main game and events have different trade offers.

It's up to you to decide if it's a worthwhile deal! If you don't think it's valuable, don't worry, they'll offer other trades in a few hours. Be sure to check out the deal even if you're just stopping in to open your timer coffin!

You can also get Trades in Events!