Forward your marketplace alerts to OnlyIf in 3 steps.
Go to Settings (gear icon) → View all Outlook settings → Mail → Rules
Click "Add new rule"
Name your rule (e.g., "Craigslist to OnlyIf")
Add a condition: From → contains
Enter the alert sender: craigslist.org or ebay.com
Add an action: Forward to
Enter your OnlyIf address: your-criteria@onlyif.email
Click "Save" and you're done.