Yes, it's a better deal on the face of it, but as soon as AcmeShop goes out of business, competition is gone and Facebook can charge whatever they want. And we all know from the US markets that low to no competition does not benefit the consumer in any way.
Facebook Marketplace is a marketplace. They don't control the products that are being advertised. All they can control is the weighting of ads they run on certain products e.g. "AirPods are popular, increase ad spend to 20% of total".
But they could do this by simply optimising for ROAS just like every other company does.
If Facebook is sharing AcmeShop data with Marketplace, and it's a very big if, it would still be of almost no value.
Facebook isn't setting prices on Marketplace -- sellers like you and me are. We're not getting any data from AcmeShop.
When you say "Yes, it's a better deal on the face of it", I don't know what "deal" you're talking about.