People don't wake up one day and think "oh hey, I'm in the market for a craigslist alternative, let's look for one". They need to be told that Yardsale exists, and it needs to find a place in their minds. The best way it can do that is to do one thing well.
As for why it's mobile specifically, look for why people do anything on their mobile phones.
From the article i get that this way they try to encourage people to take photos. This is good. But it's not worth all the market you are cutting out, especially you are cutting out people who wants to buy, they have no reason to be forced to browse from the app. An alternative would be make the website available to everyone and suggest(not force) people to use the app just to make the advert. Maybe even by applying a little "tax" to those who use the website. This way i think you would have the same effect but a larger audience.
Warning - Rant:
This strikes me as typical of two things that seem to be trending -
1). Build platform-specific (be that HTML5, Windows, or iOS or whatever) that cuts out otherwise interested users.
2). Make sure it's "social". That word right there is starting to turn me away from apps. I have enough "social".
Many apps are just brushing old ideas with new social, mobile or html5 coatings. Ignoring the fact that the idea is still the same.
Next thought: "Don't have an iPhone. Oh well..."