I would like to suggest another rule.
Introduce a time limit for filing complaints for this sort of things, for example one week or one month. Because we have inflation on our hands, and market value of items is not a constant. So, it is possible to imagine that someone complains that he has been cheated because after the patch/several nerfs or buffs, MW Shashka is worth now 2000k, and he sold it for 900k.
Anyway, I suppose this is covered with common sense rule, but I think it wouldn't hurt.