Is simple: ask for a purchase confirmation.
Last weekend I received an email from a guy asking for some help with one of my items. From question to question I discovered that he didn’t bought the item on CodeCanyon but from some other place.
I usually receive around 5 direct emails a day asking for support. So this week I decided to verify to see if the people actually purchased the item on CodeCanyon. From Monday until today (Friday) I received 27 direct emails from different people and only 3 !!!!!!? actually bought the plugin on CodeCanyon.
I don’ know if this happens to you, but the part I have about this is that some people (jerks) take you item and they sell it (they don’t give it for free, they simply sell it) as their own, but they don’t even bother to remove my name and email from the files. Some of the people they contacted me actually though they were buying a real licence and they needed to buy it again from CodeCanyon.
First thing is to let your customers know that you only answer mails send via contact form on your profile page – in that mail you’ll get link to verify if user purchased your item. You can also start support forum like many authors do.
Second – we all know your pain, and we don’t talk here about piracy
Of course is obvious but I didn’t thought they were so many.