1. Go to the original thread which the item is listed for sale. Here is an example: Selling - Summoners War Pilot Service | PlayerUp Accounts Marketplace. Player 2 Player Secure Platform.
2. On this page, you'll see on the left hand side the original poster username as seen in the image below which for this specific seller it would be Rolan however your sellers username maybe different:
3. If you're unable to find the sellers username still, enter PlayerUp as your sellers username or open a ticket here with Middleman Support.