Oooh. just love those words "tried ourselves". Of course they have made it worse and now want you to sort out their mess.
It's only one chair, in all honesty I would pass on it. The time for travel, set up, clean etc would far exceed what they could pay for a new one in Staples!
I would say to them that your price would be £xxx, but no guarantees on being able to remove the marks and suggest to them that on this occasion they may well be better off purchasing a new one. Then of course you make sure you leave all your details for them for their eot cleans etc etc