1.> In top right corner, click members
2.> Click "toggle more options..."
3.> Type in the member's name in the box under where it says "Signature Contains" and press enter
4.> If that member's name is in anybody's signature, the users with that name in their signature will pop up and you can contact that member to find out how the sale went. Also you can click the member's names who come up in search to see their profile and see what is in their sig about that member before you contact them.
Please note, not all members might be in someone sig, so don't get upset because you can't find them.