On occasion you can find sellers on
ebay that miss the mint mark, too. In a few cases it is obvious that the seller has little if any experience with coins and you can see that clearly when every coin they have listed has the date but
no mint mark on any of them. This is particularly true if the mint mark is "hidden" or not easy to see, such as the reverse mint mark on a
Buffalo nickel.
Once in a while, you'll even see "big time" coin dealers who miss a mint mark.
My rule of thumb on these is that if there is a significant difference in value between the coin with or without the mint mark, I advise the seller, especially those who are "selling their dad's collection" or are likewise uninitiated in the finer points of numismatics.
However, for those dealers who seem to have a hard time "grading" or who list every coin they advertise as "key date" or that do some similar silliness I will take advantage of their lack of proper identification and not feel badly about it. Earlier this year I purchased a 1876 CC
Seated Liberty dime that is in super nice EF 40 with fully natural tone (tone slightly dark so the mint mark was harder to see) that the seller had listed as a Philadelphia issue.
The fact that is was worth about $30 more with the CC mint mark was gravy. However, after I took pictures of the coin I noticed the legend on the obverse looked funky and the gown on liberty was messy looking at the bottom. My first thought, of course, was that I'd been had with a counterfeit, since I've never once had any reason to explore the varieties for that date or mint mark. I wouldn't have even NOTICED those differences if I didn't have the large screen photos to work with.
It turned out to be a double die obverse, which I sold two days later since SL Dimes are not one of the series that I ever hope to complete, and used that nice chunk to finish my date set of
Two Cent Pieces.