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Hello @ingoda, "Sandbox Exchange" is similar to saying it's a "Simulated Exchange". Idea being we are a real time exchange that uses real world exchange numbers, however we give you $100k in play dollars that you learn to trade with safely on our "simulated exchange"...the goal is to take the risk out of using real capital during the time newbies learn to trade. - Team Niffler.co

Oh, I see.
What's the point of adding STEEM there in that case?

Hello @ingoda... We (and at the request of many from the Steemit community) are interested in potentially adding STEEM as not only do we hope people will learn how to trade with STEEM but more importantly we want our users to learn more about what is behind STEEM ie Steemit thus increasing it's user base :-) - Team Niffler.co