I seriously doubt their business model is based on fencing stolen goods but I would be interested to hear the customer service directors excuse for why this is not the case.
I don't work for one of those apps, but have used one. I like it because it is a lower-friction way to buy and sell items. I can take pictures of items in my garage with my phone and quickly list them without going to my desktop and writing up a Craigslist post. It's the same old story of Uber/Tinder/etc making something easy to do on your phone.