Didn't they say that 200 marks is the size of your wallet but as soon as you spend them you can go and get more all week? This would essentially mean there is no real cap, you just need to spend them so you don't waste the ones collected while you're at max capacity. As far as a shared stock goes, I think it's wise. sharing marks would allow you to be leveling one character in one way but being able to buy gear with another, it's seems to be cutting down the time grinding on each individual character.