Go back to the WSO seller's (or appropriate) thread and underneath their name there is a small, red triangle. When you hover over it, it will show you that it's a "report post" icon. Click on the report post icon and submit a report to the WSO/appropriate thread moderator.

Make sure you include only the relevant facts, such as the date of purchase, what steps you took to reach the seller, how many times you contacted the seller, and how long it's been since you've heard (or haven't heard) from them.