Hi Marilyn, I agree with what you said, all are possibilities.
Now that I see the complete description though, plus the age of the book (1972), I am pretty sure the dealer is stating that the covers are lightly spotted, and the implication is it is some sort of defect. That could be from soil, or it could be from mildew, or really any number of other things.
Jesse, if I were you, I would send the dealer a question and ask them to explain exactly what the spotting is from. If it is soil it is probably not an important issue, but if it is mildew I would be cautious, unless it is extremely light indeed.
all the best, Michael (TheManInBlack) T