I'd ignore the nay-sayers and hang on to it. There are many collectors who would shell out hundreds for it. Look at what happened to the 1922 "no mintmark" cent. Maybe in another 20 years it will go up again. If it is a "filled die" variety, it would make sense that partially filled die examples might exist. What do the experts here have to say about that?
Since this thread has been resurrected from the grave it raises an interesting question about Philadelphia issues with no mint mark. The 1922 Cent has gone through the roof, selling for tens of thousands; the 1982 Dime sells for a few hundred; the 1989 Quarter sells at the same price as other contemporary quarters. The Cent and Quarter were the result of filled dies, the Dime just didn't have the MM applied. At any rate, the missing mint mark makes a great deal of difference in one case, a little difference in another case and no difference at all in another. Why the inconsistency? Or are collectors just nuts?
Grease filled mintmarks aren't an error, its a minor manufacturing irregularity. As such they really shouldn't have any value. A true missing mintmark is a true error. Btw yes, we are nuts.