I use the Market a lot to be honest.
I am Max level 60.
I use the Market every day.
As far as suggestions go:
What I would like is for the game to introduce a new tab in the market.
I have played games were some Markets have a Underhanded Back Alley type of Guy in the market.
You can talk to that person and they can do random offers or trades with you.
They will say something like:
Hey, I smuggled some new goodies.
Take a look at my stock and see if you like anything!
- I'll let you buy 1,000 Green Shards for 25 gems.
- I'll let you buy 600 Energy for 125 gems.
or
- I'll trade you 1 Rare hero for 3 Rare tomes.
- I'll trade you 1 Legendary hero for a Random Legendary hero
I have seen games do things like that.
I think it could be a clever way to help players be less frustrated with the game.