Contests for starter packs are pretty rare. That's because the starter pack codes are rare, themselves. Usually, people only get them as part of the rewards in a crowdfunding campaign. You'll need to watch out for them.

You can sell your cards through the Steemmonsters.com or at peakmonsters.com, which is a marketplace for Steem Monsters card. Personally, I use @peakmonsters for managing my cards (buying/selling, renting, etc.) because you can clearly see the card prices and how much others are buying/selling them for.