I would like to make everyone aware of mistakes NOT to make, when collecting Mercury Dimes, especially the hard to find ones.  First off, do you want a true complete collection?  Then I suggest you look in the Official Blackbook, and make sure you know what a complete set actually is. I see so-called 'complete' sets on EBay, but when you read the listing, it may state something like; "Complete except error coins", -- or "Complete except 1916-D".  Complete, -- means complete. Unless you know each and every coin in the set, you will not have a true collection. I suggest buying certified coins, but it makes for a very expensive set in the end. If you must curb your spending (like me), then it may be better to buy a partial set, and buy the harder to find (and more often counterfeited, altered, or mis-represented or mis-identified) coins in certified holders. I only trust NGC, PCGS, or ANACS when buying slabbed coins. These are the 3 companies that seem to be most consistent, and have held their value. Beware of SGS, and other un-varifiable 'grading' companies. I have collected 3 sets of Mercs so far, with 1916-D's, 1921 & 21-D's, 1942/41 & 1942/41-D's in certified holders. The rest of the coins are raw, (including the 1945 micro S's)  and if they were sent in to NGC, probably 10 to 15 coins would come back un-certifiable. When you deal in raw coins, you run the risk of buying coins that may have been cleaned. And sometimes, you cannot tell without looking under extreme magnification. So, buy the more expensive ones certified by a reputable company, and maybe buy each coin certified, one or two at a time. If you end up with a complete certified set, you have a rare gem indeed.