Few doubts on cash item and player's money

Hello, guys.

I really would like to get a little extra money to level up my skills and buy some stuff.

So, I was wondering how could I get a few more money. There is this item (check the photo) in the cash shop that costs 15stars and some people are selling it for a very high price. I would like to know if it is worth it to buy cash and that item and then sell it... I mean...Do people buy it??

I would also like to know a few tips on what to sell to get money quickly...If someone knows, please, share it xD

And just a last doubt: how do I open a store? I mean, the ones like mini-seiya.

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