If they seized his computer they wouldn't know his SR name unless he left SR logged in and the TOR browser open. The TOR browser doesn't save credentials otherwise.

You're giving the guy a lot more credit than he deserves. Beyond the fact you don't have to use the TOR browser to access TOR, it's fairly easy to change the settings to remember passwords. Maybe he was forgetful? Maybe along with his guilty plea he gave them his credentials? Maybe he used the same username and password for everything?

If they'd set up a sting involving a fake seller, it would be in their interest to publicise it. Our drug enforcement process is built around throwing the book at a few people to attempt to discourage many. If people thought that the seller they were buying off was likely to be a cop, that would significantly discourage SR buying.