I accidentally marked my eBay sales as reviewed instead of matching them to inventory. Can I undo this?

Yes, if you are on the Ultimate plan, you can do this with the following steps:

1. Click the "Transaction Details" tab
2. Select the "Sales" button to view sales:
   
3. Select sales by clicking the checkbox on the far left (if you've hidden the id column, you'll need to unhide it)
4. Match these sales by clicking the "Match to Inventory" button