I would be willing to offer my services, as a vice. Also, you could have officials to just occasionally check up on things, and return items, if something has sold.
From the list, I can say any mods would not be thieves. I would never steal. Pika doesn't seem like the stealing kind. Pyg is sarcastic and annoying alot of the time, but he wouldn't steal... I think.... Legobox may make useless posts, but he would not steal. Blockbit could possibly have scammed in the past, but there is no evidence, I don't think it was him. Dark is newer, but he seems good to me, I don't think he would've stolen. Starman seems very unlikely to have stolen from platinum, he is really friendly... Chris is often brash and likes to start fights.. I can't rule him out. Freeze54 and adam567 are complete wildcards, and the most likely to have stolen the items. I would say for one thing, no new player or player without any kind of record should be allowed to trade in platinum. That is the main precaution that needed to be considered when adding sellers. This was the major mistake, and most likely the reason the thefts occurred.
As is add a level door at the shop so that level threes and higher can shop, but not anyone else? I kind of like this idea.