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steve kThe 55 Koufax is always a nice card - even with those marks. Cards with marks "generally" sell for about 10% - 25% of an unmarked card's value, but there are a ton of exceptions to this rule depending on the type of marks and the card. Not a mark, but for instance a pinhole will automatically turn a PSA 10 card into a PSA 1. Good advice would be to study as much as you can about the hobby before investing a lot of money into it - the "studying" with baseball cards is actually a lot of fun.
Also - be careful on those NM looking vintage cards - there are a lot of reprints out there. It is highly recommended buying any vintage cards graded by PSA or SGC until you are much more knowledgable. I've been collecting baseball cards for over 40 years, and off of ebay, I still won't buy an expensive ungraded card - from anybody.
steve k