From the looks of it, I'd say it was because you have a flex and a tank in the thread. They are supposed to be listed in their respective forums.
12. Grouping items will no longer be acceptable. If you have a pile of items to sell separately, make separate posts in each respective forum. You can sell a marker as a package with other parts for one price, but please post it in the appropriate marker forum. (ex. You can't have 3 air systems, 8 barrels, 3 guns and few miscellaneous parts all in one thread in one forum.)