Your first question: Do I have to have an ebay business?
Well no you can sell your kits in whatever way you wish but ebay gives you a perfect platform to market your goods without having to have a shop. You could try to sell them online via another website: ebay is just one of them.
Your second question: Does this count as self-employed?
If you are buying in kits and selling them then yes this is considered trading. If you do not set up a limited company you are then considered to be self-employed. You need to register for tax with HMRC and tell them you are self employed. You will start paying class 2 national insurance contributions at a flat rate of £2.65 per week and then class 4 at 9 % of your pofit if it is above the £7,605 per year. You also pay income tax at 20 % if your profit uses up your personal allowance.of £9,440.
Working Tax Credits
You would have to certify to HMRC how much work you put in to your business. If you are trading online and your volume of sales is small HMRC would take a lot of convincing this was a full time occupation, but you may be able to convince them you are doing the minimum of 16 hours work a week. I would suggest you keep a log of your working hours in case of a query and be able to prove what you were doing during that time.
What if I dont return a profit?
Well this doesn't matter. It is the same as a plumber who doesn't make enough money to pay for his tools and van. You would still qualify for working tax credits unless your earnings go above a certain amount. You need to keep HMRC informed and they will adjust the amount they pay you.
I hope this helps and good luck!