The internet is replete with the laments of people who tried to sell something online and the buyer did not show. This is not only frustrating it is robbing you of your most precious commodity, TIME; and you can't get it back.
You just want to convert your item to cash from the comfort of your home in the fastest, easiest way possible. This is what the apps and sites offer...
Yes, you are reaching hundreds if not thousands of prospective buyers who call, text, or email, say they will come at a certain time and then are late or don't show. Time wasted!
The app or site is the shopping cart / sales counter, but until until the buyer takes the money out and hands it to you, your item remains unsold.
It is rare that someone stands at the sales counter, pulls money out of their wallet, and does not buy the item.
1) No one likes to pay for something they don't have in hand
2) The buyer does not trust you (don't take it personally)
Sure you can instantly send the money any number of ways to the seller, but what if the item is not exactly as described, or you get to the buyer and the item was sold to someone else, or worse the seller and item disappear on you. Time wasted!