This concept is common sense to many people, but I wanted to make sure to share this with any new players who might not have figured out how the market works when it comes to selling Leader cards. By the time they figure out it may be to late.

When you get to level 5,10,15, and 20 do NOT level your Leader card upto 5* (stars). There are many people willing to buy Leader cards that are 1*-4* easily for about 750clintz! Once the card is 5* it is only worth much less. This is because there are "rich" players who buy the un-upgraded card just to get the experience from leveling it up. Here is a breakdown of approximately each of the first 4 leaders works.

Level 5 - Timber: 1-4* sell for ~750clintz. Then re-buy (if you want) at 150clintz = 600clintz
Level 10 - Vansaar sell for 750, rebuy at 175 = ~575 clintz
Level 15 - Hugo sell for 750, rebuy at 350 = 400 clintz
Level 20 - Bridget sell for 750, rebuy at 400 = 350 clintz

After Bridget the leaders do not make any more profit, but you could make an easy 1800clintz from doing this. This helps especially at level 10, because you could get a nice 1000-1500clintz card for your deck, when you need it most.

(always check current market prices before selling to earn as much as you can)
(it will always take a few hours for the cards to sell on market, just be patient, they will

I hope this manages to help atleast 1 person. If you are in a guild which has lots of low level members, make sure to let them all know. Thanks.