Back in the days when ebay was still paying lip service to being partners in the good stewardship of philately, this was their policy on marking stamps:
All forged stamps, facsimiles, and reproductions may be listed but need to be clearly and indelibly marked as such on either the front or back of the stamp. See "Additional Information" for more details.
[policies/currstamps from November 4, 2008, courtesy of the Wayback Machine.]
ebay's Selling of Stamps
page said (on October 13, 2008):
eBay does not permit the sale of stamps that are fraudulent or improperly described. Sellers who knowingly fail to disclose information about the authenticity of a stamp, alterations to the stamp, or other information that might have a material impact on the stamp's value may be reported to the American Philatelic Society.
If the stamp is a forgery, fake, reproduction or facsimile:
it must be clearly and indelibly marked as such
a clear image of this mark must be included
it must be clearly stated in the title and/or description
If unsure of the origin or authenticity of the stamp, please state this clearly in the listing. Do not sell an item "As Is" regarding its authenticity.
Violations of this policy may result in a range of actions, including:
Limits on account privileges
Forfeit of eBay fees on cancelled listings
Loss of PowerSeller status
I'm going to stand behind the practice of marking bad items; it's good stewardship for the hobby. I'm going to assert, without evidence, that countless less-experienced hobbyists have benefited from markings on bad stamps.
I'd prefer it if people didn't mark good items, even with a pencil. That said, I've benefited by it.
Many U.S. dealers will have Commonwealth material that they've identified through the 6-volume Scott. The old penciled-in Gibbons number means nothing to them. I've also flipped over a stamp that I was contemplating for the centering, only to see a penciled-in perf variety. That can take the item from "might buy" to "must buy" in a heartbeat.
So, am I a hypocrite for saying I don't like markings on good items? I usually
don't, and I've never added one, myself.