So there is a generous bootstrapping period.
This is obviously untrue. Walk into almost any store in the world, and you're in a marketplace where the owner is not also selling his own competing products.
There are vanishingly few exceptions, like garden centers, big box stores, and supermarkets.
#1 Amazon selling third party products.
#2 Third party sellers selling their products.
#3 Amazon selling their own products.
You're pointing out that #3 isn't necesary, and that's true, but my point is that #1 is necessary and is how every store starts, then they might expand into #2 and #3.
At a store (once again, generally speaking), the company buys things and re-sells them. In a marketplace you're buying directly from someone else (and they usually pay the marketplace operator a cut of sales afterwards, or an upfront fee to sell there).