Maybe you could based on what Guitar Center and Best Buy sell cables for to the public but it would probably be difficult to do it for less than some established firms selling online or for the actual dealer costs.
Perhaps this is all familiar territory for you, but when you get into selling products then along with the direct product cost there are also numerous indirect costs such as maintaining a cable and
connector inventory (you would likely routinely have to
purchase raw materials some time before selling the related end products), returns and warranty issues, samples (at least until you are a 'known' many people will probably not buy anything without seeing a sample) and so on. Then here is dealing with sales tax and receiving payments.
Looking primarily at only truly custom cables might be a
bit better as many of the production efficiencies that make it possible for some companies to mass produce more 'standard' cables at a lower cost would no longer apply and they might actually not be at all cost effective for '
one off' products. The flip side is addressing issues such as a custom cable which is returned or canceled after you already built it and that may not be easy to resell.
You might find are some local Contractors that need custom cables on a fairly regular basis. A sister company of one I worked for had a couple of big projects where they had a couple of full time people just to make the custom cables involved.