Austin's 3:16 shirt might be the greatest ever (taking your word), but Cena has about 18 (arbitrary number) different shirts, wrist bands and the like that add up to greater than anyone. Not to mention, he's been selling those for over a decade, and while you can still get an Austin shirt, they're not going to sell nearly as much nowadays or really even just a few years after he left. Place that on top of the fact that he was only active for a short period of time.
Per Meltzer, in 1998 Austin's merchandise sales numbers were $13 million (adjusted for inflation would be about $18 million today). Forbes estimated that in 1998, he sold 12 million shirts, and his merchandise accounted for 50% of the WWE's total revenue for the year.
Someone's numbers are obviously off a bit, because you weren't buying a shirt for a dollar back then (more like $20), but the point is simple. Austin sold a shit ton of merchandise. For a point of reference, the best year I could find for Cena was 2009, and he sold about $5 million in merchandise.
Yes Cena does sell a lot too, and has been for a long time. And yes, he also has a lot of various things with his name on them being sold.
Thing is... so did (and does) Austin.
It wasn't like he just had the one shirt. He was constantly coming out with new ones. Plus hats, vests, and various other things. One of the most popular items on WWE shop zone right now? An Austin 3:16 beer cozy. Austin was huge on merchandise because that's where he made most of his money (still does). He was no different than Cena in this regard. The major difference between them, is while Cena's never really had that one iconic thing, Austin did. Poll 10 fans on Cena's most iconic shirt, and you'll get a few different answers and a debate. Poll 10 fans on Austin's most iconic shirt, and you'll get a unanimous choice on one item.
Sure he doesn't sell nearly what he did when he was active and the top guy in the company... but he still sells on WWE shop today, and they're actually still coming out with new merchandise for him. In fact, went to shop zone, sorted their t-shirts by best sellers (not sure what type of date range they use), and saw a couple interesting things.
The Austin Podcast woman's tee (new item) and Austin 3:16 are both outselling anything of Cena's (Cena 15x shirt is currently the most popular Cena item). And then the Austin What? shirt is outselling everything else.
I also noticed that holy shit everything looks the same on there (and mostly resembles to varying degrees the type of stuff Austin was coming out with 15-20 years ago.
The point there, is that a guy that hasn't wrestled in 13 years is outselling a guy that hasn't wrestled in in about 4 months. So I don't think it's really fair or accurate to say that Cena wins the merchandise sales category by 'an enormous margin' as Tastycles said.