This won't help you with a sofa, but here is something i learned and thought I'd share. We bought a new oak kitchen table, and DD2 sat down at it the first night with a Sharpie. We realized she was being too quiet, and when we looked over at her (we were trying to rearrange the rest of the kitchen to accommodate the new table) she had scribbled all over the brand new table...literally like 3 hours old! But, much to my amazement, Mr. Clean Magic Eraser took it right off with no damage to the table! I was so surprised! On that same table just a few months ago, I dripped paint from a paint pen on the table, and the Magic Eraser got it off as well!
I know that won't help with fabric, but thought it might help someone else with marker troubles!