I don't buy the whole "Cena sales more" argument and never will until you provide me proof but meh, I do agree with what IC says.
First, I'm going to take you back to 2004. The WWE made $56,400,000 dollars on consumer products (clothing, DVD, games, general merchandise etc.) Of that $56.4 million dollars, John Cena related merchandise exceeded $12,000,000. This was back in 2004, with Cena as a midcarder and holding the US title AND on Smackdown. For those not very good at math, that means that John Cena, as a midcarder, sold more than 20% of the WWE's merchandise in 2004.
Links:
http://corporate.wwe.com/investors/documents/HistoricalFinancialCharts2-23-07.pdf
http://www.wwe.com/superstars/raw/johncena/reviews/3481716
The first one links to the merchandise revenue (on page 4), and the second one is the source for the $12 million dollars.
Now, in 2005, which was when John Cena became champion and moved to Raw, WWE revenue from merchandise jumped from the $56.4 million in 2004, up to $76,200,000 in 2005. In 2006, with Cena still holding the title, or chasing it, revenue jumped up to $95,000,000.
link:
http://corporate.wwe.com/investors/documents/HistoricalFinancialCharts2-23-07.pdf
Now, keeping in mind the fact that Cena merchandise made up more than 20% of merchandise in 2004 as a midcarder, does one really think that the nearly $20,000,000 jumps in both 05 and 06 are coincidence with Cena being labeled as number one guy? Of course not.
Hopefully that'll be enough proof. If you would like more, check out the WWEShop.com page. Notice how much merchandise is Cena's. If you understand anything about a free market economy, and the laws of supply and demand, you'll understand why Cena supporters can use the high level of selling merchandise argument.