It's unfortunate when GB organizers will not refund participants like myself who want to cut ties with a GB (most notably due to duration), because there's currently no real means to sell. According to the Classifieds Rules thread:
Do not post advertisements outside of the Classifieds and External Auctions subforums.
Plenty of users, myself included, have broken this rule. (And not just selling, but "I'm looking to buy" messages can be considered WTB advertisement too, right?) It seems like this is hardly ever moderated, but it also seems difficult to determine how effective this is anyway. Apparently I haven't had much luck.
Can you give examples of this occurring? If you're referring to selling your spot in a group buy in the group buy thread, that's actually allowed (and the recommended method), per the Classifieds Rules.
If you are trying to sell your spot in a group buy, you should do so in the corresponding group buy thread instead of in the Classifieds.
Classifieds Post Content
Only list for sale or trade items that are already in your possession.
It seems like moderation is much more stringent on this rule. The reasoning, that it helps protect users from bad transactions, is understandable.
But what makes the idea of selling GB spots unreasonable? Can this be officially allowed in some capacity?
Moderation is indeed stringent on this rule, because it also encompasses the point you're discussing, by default. If the GB hasn't completed, you don't have the items in your possession and cannot list them for sale in a Classifieds post, as exitfire mentioned.
Selling of GB spots
is allowed, as I quoted above, it just must take place in the GB thread. And it's up to the parties involved to determine how to handle payment transfers and address changes. If the GB organizer is unwilling to make those changes on their end, the two individuals can make their own decisions in PMs. The only truly unique aspect of selling a GB spot is that it must occur in the GB thread, not Classifieds.
Does that answer your question, or are we right back where we started? I think I may be misinterpreting your comments slightly, so please elaborate/clarify as needed.