Restoration yard. Try and source as much old stuff as you can, cast iron radiators, railway sleepers, metal signs, etc as cheap as you can and then sell.
You will have random weirdo's turning up though unfortunately.
All my other ideas are based around cars. Brake caliper refurbishing or a second hand parts supplier that specialises in one particular make or even specific car. I.e. Ford Fiesta specialist or whatever. Have the garage shelved with all the parts all cleaned, photographed and listed and easy to ship out quickly.