This is actually a hard question to answer, because it's hard to pick just one product. I'd say music, clothes, e-books, etc are purchased the most online.
If your looking for a business idea to start I caution you about going after the big ticket items (cellphones and what not) as there are much more people going after those items. Try and find a unique nich on something your interested in.
Electronic gadgets & travel tickets are the most selling products online globally. Apart from this there are mostly services which have been sold the most through online media.
I think that digital goods sell better than anything else online. By digital goods I mean: - Templates - Graphics - Motion Graphics - Flash projects
The most purchased items online must be cell phone covers, chargers accessories in general. Ebay pulse is good place to look. Many big and small companies look at ebays data. Amazon too not sure were to find amazons sales data though. Everything is going local mobile search
One of the most popular items are tickets - for travel, concerts, and then there are -Electronics- laptops/ipods etc...
Information I think is the most purchased thing in the internet. That includes e-books, articles, membership forums, newsletters, etc.
I generally purchase books, music cd's, dvd's, movies, sometimes bags and wallets too. and now a days there are a lot of people who are buying mobile phones and other electronics products online.