Here's a nice overview:
http://en.wikipedia.org/wiki/Usage_share_of_web_browsersHere you see that desktop/laptop is mostly using IE
http://www.netmarketshare.com/browser-market-share.aspx?qprid=0&qpcustomd=0Now with more and more tablets and smartphones browsing the web you notice that other browsers, marketed like MS once did and was fined for, are growing.
Overall I wouldn't be surprised that 'other' browsers are more & more encountered and will 'rule' the stats though on desktops and laptops it is still for the bigger part IE.
So in my opinion we will get more browser issues with people using more and different devices each with their own 'default' surf tool. With in mind the fact that browsers still not use ONE standard.....
Now, looking at osClass and how people are using online market places like ebay etc. they most likely use a laptop or desktop meaning it is smart to make sure that your website not only looks nice and working on the iPhone, iPad, Tablet or other smartphone. Why? Because imo the majority of people posting ads still use their desktop/laptop. They can sit down, take some time to think, edit, think, edit their ad text, look up some pictures or make some with their webcam or whatever. Drink their coffee, show it to their wife/hubby etc....
Though I would like to promote the development of an App for use on other devices like smartphone/tablet because people just 'browsing' for a quick cheap deal most likely use more & more their mobile devices as well as younger people are used to mobile devices and so better create an app that nicely integrates with the regular browser version and offer a speedy / easy way for the young and the furious who want to post an ad in a few clicks using their Phone/tablet.
In short, look at what people are doing on your site, when, why and how..... do not just look at stats from sites that maybe only offer information like a Wikipedia.