I'll mark a gas leak by tying orange flagging to the pipe but otherwise I don't mark defects. If I am working for the buyer the house does not belong to my client and there is no guarantee he will buy the house (especially after I get through with it). Pics are usually good enough to identify the location of a defect.

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"Baseball is like church. Many attend but few understand." Leo Durocher
Bruce Breedlove
www.avaloninspection.com