Probably because the law states that anyone who is retailing produce must legally be responsible to cover thier own costs.
By this i mean pay the tax man, as even if your self-trading you must pay tax. Common misconception is just because your trading online your no longer bound to the laws of where you live, but an online trader is bound to the same laws and regulations as anyone who opens a stall in the high-street.
(whether you declare this income is upto you, and whether you get caught not doing so is down to pure dumb luck)
As these online creditors don't have the luxury of tax evasion (as taking postal orders and cheques from people can provide), it means they have to pay tax ... this you have to pay them for the service each time it is used and to do this they need a valid bank account to attach this to so they can take thier share + the tax mans, etc, etc, etc...
Nifty in a way cause it means no accountant headaches; but still the fact remains you've gotta legally be liable if everything goes tits up.
(as your site and creditor will charge money whether you decide to send the goods or not which can lead people into alot of problems)
There are probably 1million&1 other reasons ontop of those why minors aren't allowed such accounts. You can have your Parent/Guardian however put you onto thier account and trade that way.
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