In the case of Facebook, I believe that it is don't know, don't care to know. They are selling advertising, period. Running ads for plans for perpetual; motion machines, current production name brand computers at 5% of MSRP, and various types of snake oil all for the almighty dollar. (This isn't the Marketplace but ads running in my news feed.)
In the case of eBay, I can see where it would be difficult to separate out bogus offerings from the legitimate ones. Occasionally, there are some real deals to be had for one reason or another. Given the vast number of possible items, it would be virtually impossible to create an algorithm which could spot a scam.