if your already earning above 6k(or whatever it is atm) from your current job, then anything you outside of work is also taxable. obviously a couple of hundred here and there isnt going to be noticed ie boot sale, odd job. But an ebay business with regular income will be so the tax man will want to know about it.

If at first you dont declare the earnings and later on when the business is bigger and you do decide to tell them, you willl have a fair bit of explaining to do, a hefty fine and have to pay the back taxes most likely.

If you think you are going to make something of it then i would suggest informing them as soon as you make some any money. I would also suggest hiring an accountant, but obviously you wont do that until your earning decent money.