Maybe it's just me, but it feels like a completely skewed concept to sell loot spots. Multiplayer adventures surely weren't designed for the purpose of 1 guy killing everything and then selling a lootspot? Some like doing that, some don't, and some are afraid of being scammed... for a good reason. So, how about just trying to eliminate the lootspot trading completely? There are couple options that might do it, but I think this would work the best:
If you complete a multiplayer adventure alone, you get multiple sets of loot as well.
This would pretty much eliminate the need or benefit of selling lootspots, unless someone wants to gamble on the randomness of loot. And most importantly, if people prefer the way the game works now, this change would have no effect on them. They could still sell the lootspots all they want. It just wouldn't be a 'requirement' anymore for those who really would rather not participate on such nonsense trading.