Any used electronic item is not permitted to import including cell phones.Also as you have imported more than 25 shipment , you will be called as a seller / re-seller, one day you might get a notice from sales tax or vat department etc.
Only advantage you have is, you can get phones at better price compare to eBay .If you want to use Ppobox then you have to pay import duty on it.
In very few cases you will be charged zero as import duty, depends on your luck and custom officer on duty.
Even if custom is zero, Ppobox won't let you know that it was zero, they will earn this money, as they don't provide any custom receipt.
DHL provides such document on request.
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