Hi, does the page refresh when you hit "ok" on the popup or nothing happens at all?
If it just closes the popup but nothing happens, it may be your browser's popup blocker - there's usually some icon appearing on the right side of address bar that shows that an action by the page has been blocked. Click on it and there'll be some options to proceed with the action once / allow all actions from toribash.com.
In case the page refreshes but the item isn't purchased, please let me know if there are any messages in red/yellow/blue boxes displayed on top of the market page.