As someone that's running their own business and has ran one before also, I'll offer my perspective.
But before I do, I'm not being negative, I'm being realistic. I hold anyone who has the balls to start up their own business with serious intent in the highest of regard, but facts are facts, and you need to know them, and not just have sunshine blown up your arse in the hope it's what you want to hear.
If you're selling NEW parts you need to have the financial clout to be able to buy in bulk from a wholesaler, this will mean an outlay of THOUSANDS of pounds. You'll also need a bank willing to offer you merchant facilities right from the start, you will need to take card payments (and I don't mean just paypal) to have any credibility. If you're dealing with other businesses as part of your target market and not just the public you'll also need to be VAT registered. Most businesses you'll be targeting will be VAT registered themselves and want to claim the VAT back. If you can't offer that, it will be unlikely to be profitable for them.
Motor factors are fine and dandy for same day stock availability, but manufacturers of parts vary. Plus if you're then selling these parts on, you'll make no margin on factor prices even with the most **** hot terms on your account, which you wouldn't get without references and/or buying clout anyway. Buying parts from factors for garages is fine because the part has immediate demand. Re-selling parts to the trade or public is another matter.
If you're thinking about selling second hand parts from vehicles bought in for breaking then you need to consider the various legal aspects of doing so. For a start you need a waste management license for end of life vehicles. This will require you to have premises that meet the environmental requirements which there are many including proximity to drainage, and non permeable flooring. You'll also need to satisfy those of a higher authority that you have the required waste management contracts already signed with a company that will dispose of the fluids etc. And you'll need all the right tools and fluid handling equipment to safely store those fluids in the meantime. All of that will also run into the thousands.
Even if you have all that money in the bank it's a tough market to crack, especially at 20 years of age. Credibility is everything so be prepared to make a loss or at best barely break even for 24months whilst you establish yourself and important business contacts.
So don't take that as a reason to get disheartened but it is the truth, so re-evaluate what it is you want to do, and think about it carefully before you potentially spunk thousands of pounds up the wall which could be invested elsewhere in your entrepreneurial future.