As soon as you buy a product to make money you need to declare, selling your own stuff does not fall under this remit but deliberately buying product to sell on at profit makes you a trader and liable to declare it.
deliberately buying product to sell on at profit makes you a trader
Exactly. So many people make money online and don't declare the income, really your just asking for trouble. Accepting payments by paypal or a similar system leaves every payment traceable, so don't think you can hide it.