You keep calling it this. Why?
In Oregon a recent law pushed by the anti's defines a "gun show" as any place where 25 or more firearms are "available for transfer". ORS 166.423(2)(b)
So, since a gun buyback has more than 25 guns available for transfer, it is a "gun show" by definition of Oregon law.
Then, ORS 166.438 requires a criminal background check to be performed, on a state police produced form and through the state police, for firearms transferred at a "gun show". That form must be maintained by the transferor for a period of five years. The ONLY exception given is if the firearm is transferred TO an FFL.
Then, ORS 166.438(3) requires that a person who organizes a "gun show" must post the requirements to use the form and conduct the background check, as well as provide the form. ORS 166.438(5)(a) makes violation a Class A Misdemeanor for the first two offenses and 166.(5)(b) makes it a Class C Felony on the third and subsequent offenses.
As the organizers of the buyback expected more than 25 guns (they had over $6,000 in gift cards) and, in fact, more than 25 guns were transferred they took in 138 guns), they violated the law thus making it an ILLEGAL GUN SHOW.
In addition, they exposed every person turning in a firearm to Misdemeanor and/or Felony charges as the requirement to do the paperwork is placed upon the transferor in ORS 166.438(1) and (2).
Of course the police didn't enforce the law as they were in on the buyback themselves. So if you or I sponsor an illegal gun show, it's likely we'd be prosecuted, but they simply ignore the law. Personally, I think it would make a good argument in defense that the law is not applied equally to all.....14th amendment violation.
That should answer your question.