Law Enforcement knows very well how to deal with Tor, as evidenced by busts against Silk Road and even small time drug dealers as operation ononymous.
<a href="http://www.ibtimes.co.uk/operation-onymous-six-britons-arrested-police-bust-400-drug-dealing-dark-web-sites-1473713" rel="nofollow">http://www.ibtimes.co.uk/operation-onymous-six-britons-arres...</a><p>Russia also knows how to go after Tor, as seen in Malware busts and the arrest of "Paunch" malware creator in Russia.
<a href="http://geekslop.com/2013/blackhole-malware-creator-arrested-in-russia" rel="nofollow">http://geekslop.com/2013/blackhole-malware-creator-arrested-...</a><p>If individual guns can be sold online, it only means that Law Enforcement is not prioritising going after the sales of individual guns - right or wrong - they are not stopped by the use of Tor or even encryption.