Late to this one, but it's the most useful thread on the topic here, so adding what we learned building the same thing.
The 300 miles thing isn't Haron's bug. Facebook's vehicle form won't accept mileage under 300 on a used car. A tool has two honest choices: show the dealer the real number and warn that Facebook will reject it, or floor it at 300 and say so. Silently posting 300 is what gets a rep an angry buyer.
Photos: Marketplace takes 20 per vehicle listing. Page and profile posts take up to 80. Feeding it more than that is the most common reason a batch "doesn't attach". Cap first, then upload.
Joe is right about where the value is. Getting a car onto Marketplace is the easy part. What dealers actually pay for is the car coming down the day it sells and the price following the website without anyone touching the listing. That's also where the account risk lives: a listing that says $18,900 while the site says $17,500 gets reported, and reports are what draw reviews, not volume by itself.
On cadence: the reps who never have trouble post the same handful of cars at the same time every evening from their own browser and their own login. The ones who get restricted dump 40 cars in ten minutes from a shared account. Any tool that lets you do the second thing will eventually cost you the account.
Disclosure: I'm with TurboSell. We make a Chrome extension that does this across the dealer's own website and about twenty listing sites (CarGurus included), posts to Marketplace, Pages and profiles from the rep's browser, and handles the sold/price sync above. Free to start, no card. Not here to sell it in Haron's thread; happy to compare notes on the CarGurus scraping side, which changes under you every few months.