Another tip that might be useful. If you buy cards, do it using https://sm.steemify.me/ you will get an instant 3% cashback if you pay with Steem or SBD. Especially if you buy higher level summoners (which are usually way more value compared to buying single cards) or golden cards it makes a big difference.
I just bought a golden furious chicken (best card to get +10% extra DEC rewards)
Also, if you sell cads do it using peakmonsters as they undercut the fee that steemmonsters takes when doing it from the steemmonsters site.