It will be very interesting to see how they apply this, you have always been supposed to pay tax on anything you buy and sell for profit.. But if the IRD started enforcing it in mass on trademe especially for hobbiest breeders they may be surprised by the numbers.. For example if you had a small fish room set up and sold a few fish to cover costs and it was determined a business then as Phoenix has suggested you would be able to claim business costs, and as stated above after taking into account internet connection, portion of rent/mortgage for office/fishroom, cost of fish/fish food, computer even gas/vehicle operating costs to go and collect fish drop fish off get gear etc you could find the "costs" being far higher than the actual "income".