Step 1: Write a business plan that sets realistic short, medium and long term goals, and involves a procedure for reviewing those goals.
Step 2: As Eddie says, contact manufacturers and see if you can set up an on-seller arrangement.
Step 3: Make sure that you have sufficient capital behind you, or some other regular source of income to fall back on for the first year. You will need a sum at least equal to a living wage.
Step 4: Keep records of everything that you do for later examination and revue of your business plan.

These are the basic requirements for starting up any sort of business. The only thing that alters is the product or service you are supplying.

Are you in London, or elsewhere? If you are free to set up anywhere, find out where ASDA has one of its 3D print shops and set up nearby. Once people start seeing what ASDA is producing, they'll jump on the bandwagon pretty quick.

Old Man Emu