I think the idea is that it means that anyone selling before the harvest is doing so illegally, and the nuts are maybe stolen, because they couldn't be on the market legitimately. So if you were caught selling, you'd be subject to arrest. If there were no ban, it would be easier to fence stolen nuts.

I think it also prevents crimes of opportunity, since you'd have to hold on to the nuts until you could legally sell them.

So if you went to somebodies farm in the middle of the night and loaded up a semi, you'd have to stash that semi somewhere for a month rather that selling it the next day.

For a more urban example I live in a city and there are scrappers who driver around in pickup trucks all over. It is commonly known that some will steal things made out of metal from your yard unless it's bolted down. They also steal copper wire from building sites, etc.

Now generally by the time someone notices that the scrap is missing the person has already turned in their scrap for money. And if the police figure out who did it and where they took it, the evidence has already been shredded and mixed with similar metals.

If you had to hold on to scrap for a month before you could sell it, you'd be way more likely to catch people red handed.