I did know as it informs my purchasing strategy: if there isn't a huge difference I buy from eBay (as the big sellers are on both) but if there is then the wallet takes precedence.
I think my oversight comes from my brain's refusal to believe that, in a world with checks and balances on monopolies, the (I assume) largest seller of secondhand books wouldn't be able to buy the (I assume) second largest. That could never happen, right? Right? See also most of Metas purchases.