You can start an 'online vintage electronics resale site', but as you may have already figured out is that most (small) businesses focus on a specific category, for example vintage audio equipment, vintage computers, vintage console games, obsolete test and measure equipment etc. I am active in my professional hobby/business for well over a decade now. I can say that it's not easy to make enough money (from scratch, how I started), but it's one of my passions. The trade of old electronics is also full of 'pitfalls'. You'll need to know exactly what you can easily sell and what may need (costly) repairs or where you shouldn't want to burn your hands on. Over time you get experience and start to learn the tricks of the trade. You'll also need to go into lots of technical and some legal matter.
By the way, eBay is not your competitor, because they are in the business of earning fees with online market places, not in the one of trading old electronics. eBay attracts both private and commercial electronics sellers, some of them also have websites.
What do you mean with 'diversify our product'? To paint it and glue some diamonds on it to make it sell or what? 😊