Some idle thoughts inspired by my realization that marketplaces compete for buyers, but not sellers (generally):

1. Each additional buyer on a marketplace increases revenue. Each additional seller, generally, does not. Depending on the marketplace, add'l sellers increase costs.

2. The exception is brands. Adding Nike or Apple to your marketplace increases revenue per customer and may even add customers who would not shop your marketplace otherwise.

3. Marketplaces are network effects businesses...sort of. Facebook seeks to add every person on the earth, but does Amazon really need yet another Chinese seller selling garlic presses? If the presses are not made from food-grade material, this add'l seller is a net negative.

Further, their inventory, while Amazon may profit from it, needs to be stored, clogging Amazon's fulfillment network.

Actually what these 2-sided marketplaces want is as many buyers as possible with as many sellers as is needed to fill all the wants of those buyers

and to drive those prices as low as possible.

This implies, to some extent, that the "Amazon landgrab theory" holds some weight. In other words, barriers to joining Amazon are going to continue to rise. Accounts with long history of no review manipulation etc. are valuable.