This scam is similar to the Fashion Show Scam.
A scammer will host a fashion show (usually with a large prize) that appears normal until the end. The final (or a later) theme is to match the host's outfit, and they're wearing an obscure item. They offer that item on their list during this round, but you must overtrade to get it. Once you trade a rare item for their obscure (but not usually rare) item, they lock their den.
- A scammer may have a helper in the show who faces off against the finalist. All of the items in the scammer's outfit are fairly common, except for the one rare/obscure item the scammer is offering. The helper has most of the items, including this obscure item, but not all of the items. The host says you'll win if you trade for their obscure item.
- A scammer may offer several items in their outfit for overtrades, not just one.
How to avoid this scam
If a theme is to match the host, and they offer their items for overtrades, this is a scam. Don't trade the host, and if no one trades the host, see what happens. It's almost impossible to tell if this kind of fashion show is a scam until the end, unless they require items early on.