Right now I sell spare parts salvaged from home appliances headed for scrap, it is surprisingly profitable, so much that 2 people can't keep up with it (having full time jobs, though). Selling to small repair shops and directly to end users. Would be a great full time business, but unless an investor is interested, it will take a while.
Clearly I'm wrong, so take this comment as a "I'd love to learn more if you're willing/able to talk about it"
I never sold directly but I did work for a company earlier in my career that served high amp tractor trailer generators, power distribution equipment (later the department I ended up bootstrapping for them), lighting, and grip equipment to the film industry.
Early on my time there I was tasked with unloading and either stripping down or full on demolishing multiple 50 ft tractor trailers stacked to the ceiling loads of old stands (scrap metal), lamps (scrap metal, wiring, ceramic housings, aluminum housings, electronics), and high-amp ballasts for old Xenon and HMI lamps (a lot of electronics including some pretty hefty transformers. I have a stupid story about almost killing myself crippling a 20year old capacitor in one of those things...).
I worked on that for at least 60hrs a week for many weeks at not a great wage but they willingly paid me a lot of overtime to do it.
A lot of those parts weren’t going into new products but repairing high value equipment that the longer it all lived the more profitable it was. They’re probably still using some of that stuff.
I would have never thought to have started a business doing it though! I think most places are more willing to exploit young employees for it.
Just like we'd use a bit of code to glue software utilities together, the same concept exists in hardware. Build simple interfaces to connect bits of hardware that were never designed to work together. A common, and cheap example are the USB to Serial converter dongles.
It's a great way to fix a 40 year-old machine that you can't get spare parts for: figure out a way to interface it to a more modern controller at a small fraction of the cost of a new machine.
-Is selling to repair shops a big part of your business? If not, why are you doing it? Seems like it might be a time suck. You might be better off just putting all parts online and letting those small repair shops buy through that.
-Can you hire someone to do the grunt work (pulling parts etc)? I was always worried about hiring someone and then I realized that 1. you don't have to hire them full time, many people want part time work and 2. if you are profitable enough, they should be paying for themselves and then some. So don't be afraid just to hire some local labour.
-Why do you need an investor? Run the profits back through the business or put in some personal capital. $5000 will get you a part time employee working 25 hours a week for a few months.