This was precisely one of the reasons why I didn't make a "TF2WH" when I was making Skial Middleman. I already thought of that idea and didn't want to be associated with the downsides.
By buying people's stuff, you are left holding the bag when there is:
- A new dupe.
- A once rare item suddenly flooding the market.
- Scammers laundering items.
You can either deal with this by printing more credits and deflating everyone's currency, or swallow the loss. Also TF2WH suffered major vulnerabilities in the first few months that allowed people to generate unlimited credits and the entire inventory was emptied out twice.