I agree with Mike here. What we collect is part of a production process. Not "fine art".
Do you know if a certain stain on the art is from the printing process or coffe from the artist or the collector you bought it from ?
Do you know who wrote what note on the art and when ? Or whether the art was cut on a certain side ? And by whom ?
I doubt we will ever know all the answers.
We are all on the same page that a marker or ball pen should not be used, but pencil on the back is really not a problem.
If we want to convince the person that started the thread to solve it otherwise, that's ok, but us taking it to the OCD level with pencils on the back, is a bit of a stretch.
To all that claim they would not buy art if pencils are on the back, have you really asked the dealers you buy from to send you a scan from the back or the person on eBay to send it to you ? I doubt it ...
But look everyone is entitled to their opinion.