The first problem is that the forum in question ends up getting a bucketload of new memberships, most of which never show up again after October 31st. The next problem is that often these new members aren't there to participate in the forum in the preferred/standard way, they don't usually participate in the wide range of discussion… most of them don't want to, a lot of them are there to seek props for sale, some not bothering to search the marketplace to see if someone is actually selling the item they're after.
The fact that people are visiting the board in the first place and spreading the love of Ghostbusters around is a great to begin with.
They aren't all there for that, some are just there to get a kickass prop for Halloween.
Another part to that I imagine, is the fact that some of them are after the instant gratification of buying a awesome prop to wear on Halloween, whereas the members of Proptopia would prefer that person to have a bit of an investment in the costume by making it themself.
There are at least two other forums the prop buyers can visit to try purchase something, and then there's eBay. They're not significantly hard done by from being kept out of Proptopia.
I'm sure Sean has considered the other group of people, and has viewed locking them out as the lesser of two evils. It's his forum, he retains the right to do so if he likes.