Andrew,
I greatly appreciated your response on this, and you are right, being hammered incorrectly by anybody is just as wrong as the wrong-act itself. however, there should be some provided method and means of reporting such in appropriate activity, allowing the original author to produce proof that the works are theres. The previous approach you were using to approve extensions was one way to thwart this, but now that you have starting charging the modders for certification, you dont have that see more option. However, what option you do have is running a DIFF on the two file sets and comparing the likeness.

As stated, if member-1 uploads product-A
then member-2 purchases product-A there is record of this activity in your database

then member-2 releases a mod uploads to the database which triggers member-1 to lodge a complaint

now i know this was not your intent to settle these types of battles, but by offering the market service, you are responsible for a certain level of control. now be that as it may, you may well wish to implement a security featured mod offer providing those who wish to pay a little more for a guarantee that their work will not be stolen, and if stolen the reported mod on the market would be removed immediately for investigation. not saying one or the other is right, and for each challenge charge a small processing fee. if accused is found guilty, then absolute remove from our community never to return. or if he wishes to have forgiveness, incorporate a forgiveness tax charging an x amount and providing the original author x % for having the emotional duress.

sounds crazy, and may be, but if there is no control and you are trying to run this hands off, then the better approach would be to shut the market down. yes just shut it down, that way you are free of dealing with any such reports.

Regards,
DosDawg