Buying dumb things using allowance money

My son, who is 8, is getting an allowanca and has accumulated some
money.

Now he wants to buy something that is, in my opinion, a complete
ripoff, which is a pack of "Club Penguin Trading cards". He wants me
to buy them online and he would repay me in cash. The deal is $49 for
a pack of 50 cards. (!!!!!!!!!!!)

My first instinct was to just refuse to do so, but now I am having my
doubts. The cards are not really harmful (as would be if he asked me
to buy cocaine for his money, for example), they are just a dumb way
to spend money. But maybe this is his prerogative to do with his money
as he sees fit, as long as no harm is done.

On the other hand, if, say, he would want to participate in some kind
of fraud, like a pyramid scheme, I would be right to refuse. These
cards are just one step away from fraud, so maybe the refusal has
merit.

Thoughts?

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i