I wonder how they are planning to do this because last I checked you can't transfer iPhone apps from one account to another without losing all app history. Last I checked, Apple's only solution is for you to delete the app and then republish it under the new account. This means you lose your rank and users who downloaded the app under the old account won't get updates unless they get it from the app store again. If there were a way to transfer apps while keeping history and ranking, I might consider selling some of mine too.

Perhaps they can transfer the whole account. (UPDATE: Indeed, I see the Apptopia listing says the sale includes the iTunes developer account.)

What happened with DrawSomething across the Zynga acquisition (or any number of other higher-profile acquisitions)?

I sold my Android app on Apptopia and it was really a great experience. I spent 6 weeks developing a really nice, 100% original Android game that I thought was great. I had some IAP and an ad or two, but I never made more than $10 from the game. Nonetheless, I had 10k+ downloads and it was certainly a high-quality effort. I thought (and still think) that there was money to be made from the game I just didn't have time to execute. I listen on Apptopia with a high price of $2500, being totally honest about my ~$0 revenue so far. A few months later, I had an offer and my app was sold. The process was easy and I'd recommend it to anyone with a side project they don't have time to maintain. Also I'd like to personally vouch for Apptopia's founder, Jon. I have been in email contact with him since the early days of the website and he will work incredibly hard to sell your app if you list it. He personally found my buyer for me and made the deal happen.

Shameless plug: I wrote something really similar using Meteor, so the drawings are realtime and you're not limited to the iPhone (but works on it too). I was thinking of adding chat functionality too... Since it works on the desktop, it actually has legit uses like sending previews of works in progress with annotations. Would love to get more feedback though: http://my.sket.sh/

You know, I wish I'd thought of auctioning off my side projects. If the market supports it, why not!

I was thinking the same thing, sort of 'follow up starter' kind of model. In non-virtual businesses its common to sell a small business from one owner to another because the original owner wants to do something else, my neighbor bought a bakery that way (not advised if you're not a morning person!) why not technology companies.

What would they sell something that's getting traction?

Have you considered reading the article? The full title as well as 50% of the article addresses exactly this.

Maybe they believe in the next thing so much that they are willing to let it go. Maybe they are sick of this project. Maybe they don't think that it has much of a future after the initial surge of users. It's all speculation though.