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Marc S. Ebay's pre-1950 category had exactly 3,466 items listed the last time I checked.
Of those, 48 had titles and/or descriptions that contained the word "trimmed". (this includes a handful where there was a guarantee that the card was not trimmed or altered in any way). 22 had titles/description including the word "short". There is some overlap here.
So, there are about 60 cards that are listed in the category as being trimmed and/or short (a search on "altered" turned up nothing significant). This equates to a little over 1.7% of the listed auctions.
Anyone care to hazard a guess at what the actual percentage is?
I was also going to any of the dealers about the percentage of altered cards they come in contact with, but then I thought that that may not be an appropriate question. From my experience, the vast majority of the time, a collection either has a significant number of alterations, or very, very few.