You can't take a part of the transaction since they all happen in-person with cash.
Not sure about Yardsale. None of the people around here knew about it. There's definitely a chicken & egg problem in this space. But I could see these things spread through small communities first. I had hundreds of registered users in a couple of weeks from Utah and watched it spread to Idaho and Arizona pretty quickly.
Well, eBay allows people to sell things to each other across the world/country which solves a different problem. Most of the people selling things on classifieds have no interest in having to go to the post office to ship your item and deal with paypal or something to get money. The items on these yardsale groups sell within minutes and someone comes to your door with cash in hand and the item is gone. People really like and trust the community aspect of selling things this way.
Plenty of people buy and sell on ebay within small local communities with a pickup only option. The trust factor comes from being in the same community and doing at least one good transaction. The mode of payment has nothing to do with it. In fact, it would be better if you didn't have to deal with cash.
Just because things have been done in a certain way doesn't mean they can't be done better.
The Yardsale app seems quite good, but no one uses it here in London. Is it popular in SF or other parts of America?