People look at eBay and assume that eBay became the world's largest marketplace for pretty much anything (as long as it's legal to trade) by attacking all these categories in parallel. When they then try to build "the next eBay" they fall into the trap of doing just that: Addressing each and every niche at once. And fail as they are trying to boil the proverbial ocean.
And yet - that's not how it works. The way it worked for eBay was: Start with collectibles (a market which was highly affine to the whole flea market idea as the buyers and sellers were doing it offline for decades). Have a "Other" category. Monitor this category for trends and once you see enough momentum behind a specific niche, build the infrastructure to cater to that niche. That's how eBay became the largest electronics marketplace, a fashion retail powerhouse and the biggest car dealer. If you would have asked me in the early days if we could sell cars or fashion on the site - I would have declared you insane.
This is what's called a "Bowling Alley Strategy" (a term coined by Geoffrey Moore) - start with one pin (one category, niche or vertical), penetrate this segment and then move on to the next pin. In time they will fall - one by one. :)
Boiling the ocean is incredibly hard (and recipe for failure). Going niche after niche after niche - that's how you win.
(*) You can replace eBay with pretty much any successful, established company. This pattern repeats itself. Take Uber: Started in one market, then another, then another...