An actual shop would die on it's arse.
Look at Halfords. They pretty much run 3for2 constantly and imagine the quantities they can buy in at. You'll never match that.
If you want to do it, start small. Get a website. (Pay to) advertise on various forums. Get on DW. Import micro fibres for China (read Costco) at tiny pence each. Bit of this. Bit of that. Max a credit card get £10k's worth of stock from Meguiars or something. It's risky but if you believe you can do it, then crack on.
Then make sure everything goes out FAST! If I order today, when you get home tonight from work you're packing it up and posting it out tomorrow. Good customer care. Have a place where people can ask advice. It might cost you time but if you helped me, I'm more likely to use you again. Customer service/keeping people happy is far more important in a lot of cases than the price.
I bought a HU adapter lead from MT Audio last month. I PM'd him and bugged him with questions, he helped. I fitted it (wrongly) and bugged him with more questions. Each time he was quick and helpful to reply. Got a good price and it was sent out very quickly. Absolutely zero complaints. Now if anyone asks me about buying anything audio related, where am I going to send them? That's where you need to be. Keeping people happy.
I think it was i4detailing that used to, maybe still do, stick a little packet of tiny sweets in with the order. Those sweets probably cost him 20p a bag at the most but it's customer service and it's a nice little surprise when you open your box.