From everyone I've talked to (and it seems like someone calls every day with the panacea) there are two things that they're basically offering
1 - They are manually posting through your account (you give them access to your personal/business account, and they basically "manually" post the ads - or they say manually...) the issue is most dealer's I've talked to (including myself) cannot access marketplace from their business page to start with, making it a non-start, even if you wanted to share all that info/data with some company.
2 - They are posting your inventory through personal accounts. Some are bots, some real people. Basically what everyone is doing themselves once Facebook locked them out of making marketplace listing under their business page. This can work so long as nobody reports it (or if they do report it, if Facebook fails to do anything about it, which is decent odds) but of course, at some point they will likely crack down on this practice.
* It is important to consider state regulations with advertising with any of these but especially having your inventory posted under a personal account. If the State (wo)man comes to your dealership and asks to see the salesman license of one of these accounts posting your inventory, you are likely on the hook for a lot of $$$ in fees/fines.