Thanks for explaining. I think I've identified the part in your argument I don't understand:
> but they [Facebook] control which ads they are showing, and naturally with Marketplace-only placements Facebook has an advantage over AcmeShop
My question is, why do you think Facebook won't simply sell the ad space to the highest bidder, whether it is AcmeShop or a Marketplace seller? Why do you think Facebook is giving an advantage to Marketplace ads? Do you have any evidence they are?
Remember, Facebook doesn't even make money if you sell something locally and pay in cash. A big way that Marketplace makes money is through ads. But if AcmeShop is bidding more for ad space, then presumably Facebook will show the AcmeShop retargeting ad instead.
If there is evidence that Facebook is showing lower-bidding Marketplace ads over higher-bidding AcmeShop ads in your Instagram feed, because of the fee it takes on shipped items that use Facebook Marketplace Payments, then yes I agree that would definitely be an abuse of power! And thank you for explaining that -- if that is what you're saying.
But I think it has to be shown that Facebook is actually engaging in that kind of anti-competitive behavior. We can't just assume it. There's nothing inherently wrong with running the two businesses together.
(Also, if by "Marketplace-only placements" you're referring to boosting listings within Marketplace as opposed to more general ads like on Facebook, there's nothing preventing AcmeShop from listing and boosting its items on Marketplace as well.)