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Gambling has a specific economic definition, referring to wagering money or something of material value on an event with an uncertain outcome with the primary intent of winning additional money and/or material goods. Typically, the outcome of the wager is evident within a short period of time.

A great Definition of what Gambling is. (Look for others goolgle "define: gambling")

So lets see if that Definition fits on Remi's Rare Find's.

wagering money or something of material value
U can do both, u can wager a fixed amount of SparkCash or 3 Items u won before.

on an event with an uncertain outcome
It is fully uncertian.

with the primary intent of winning additional money and/or material goods.

Yep the only intend is to win material goods, the payshop items.

So it looks like Remi's Rare Find's is indeed Gambling.

Edit:
So the next Question is if its a form of Gambling that is appropriate for kids to play.