Repairing iPhones and reselling - taxable income?

Hi

I've been buying, repairing then reselling older iPhones (5-6s generations) and was wondering - should I be paying tax on these? Most of them are broken bought off eBay and then sold to a local repair store that will sell them themselves. Is the income, or all the money I get liable to income tax? How do I pay it if so? I only work part time so have a little bit yet until I hit the basic tax band according to this salary tax calculator, so I assume I'll be ok for now. Anyone got any good sites for me to have a look through in the mean time?

thanks in advance