"You earn on selling too don't you?"
Not at all.

Only buying counts towards earning DEC that gives us the money to buy stuff for our guild. We have designed a ton of other free features simply to attract people in hopes they'll stay loyal and buy cards. What market is used to list has very little to do with what market is used to buy... however we like when people use us to list because then maybe it helps our users that have bids so that we can attempt to win them a contested low price listing for their bid. But also we warn our users that list that they may be listing too low and users who list on our site can see what the bids are for so they usually list at a better price. Users who use other markets are sometimes screwed and make lots of dumb mistakes and our bid users take advantage of that a lot.
If sellers want to list their cards at a lower fee % that can help... while it ends up being less money in our pocket some markets only list cards at 5% thus the listed cards won't be shown on steemmonsters market interface for example and that is probably better for us.

What do you mean "investment" accounts?