So, how do you differentiate between those confusing mint marks to avoid paying a high price for a low value coin? At the outset I suggest an excellent reference for photographs, "The Official RED BOOK, a guide book of UNITED STATES COINS", available on eBay, in bookstores, or in almost any coin or hobby shop. Allow me to state that I DO NOT sell this coin guide, lest the reader surmise that I am attempting to profit from this article. Quite the opposite is true: I wish for YOU to profit (or at least not get taken) whenever shopping for this coin.
Concentrate first on the 1979 specimen. Keep in mind that it must be from a proof set as these coins are the ones with the different types of "S" mint marks, not the coins in circulation or those found in mint sets. (This is true of the 1981 specimen too.) As we all know, proof sets were made in San Francisco during this period, so these coins will have the distinctive "S" mint mark(m/m) just to the left of the portrait's collar as you face the coin. Unless you have very keen eyesight, obtain magnification (7x is sufficient) to inspect the m/m. What do you see? If it appears to be a glob of metal vaguely resembling an "S", your coin is a type 1, or as they are sometimes called, filled "S" dollars. But what if the m/m actually looks like an "S"? Eureka! You have a type 2 or clear "S" dollar which sells for quite a bit more. (Price of course is determined by supply/ demand.)
Let us now consider the 1980 specimen. All mint marks to date on this coin are of the clear "S" found on the 1979 type 2 dollar. If you notice a different design in your searching, you will perhaps have discovered a major find!
The 1981 proof seems to be the most confusing and also the one which causes buyers to make most of their purchasing mistakes. You now are familiar with the clear "S" m/m found on the 1979-s type 2 and the 1980-s dollars. On the 1981 it becomes the type 1 mint mark, so what is scarce on the 1979 is now commonplace on the 1981 (and 1980). Both mint marks are CLEAR on the 1981 dollar, so to better understand the difference between the type 1 and type 2 mint marks, imagine that you are standing on an over-sized dollar and straddling the "S". The top of the type 1, the one we are familiar with, is rounded and smooth in texture, and its top and bottom ends, or serifs, are rounded and rather nondescript. (No goldmine here!) However, the type 2 is noticeably different once you know what to notice. First the top of the "S" is NOT rounded but somewhat flat, and its texture is grainy and has the same frosty appearance as the portrait. Second, the tips, or serifs are distended, (and look similar to a swollen big toe, if your feet look like mine!) as though the designer had some metal left over and just tacked it on to either end. The top serif is the more distended of the two and will serve as a good pick up point (pup) when attempting to discern the types of mint marks. Once you can discern the differences in the mint marks on Anthony dollars, please know that the distinction also applies to the other denominations in the affected sets, though their respective price differences are not as magnified.
So now, when someone attempts to sell you a 1981 proof, type 2 SBA at a high, high price, advertising it only as a clear "S", you will know that he either doesn't know the difference or is betting that you don't, or both. If you were formerly beguiled by others' deception or ignorance, you no longer will be, because you now have KNOWLEDGE! A caveat if you are buying sight unseen: be especially leery of sellers who have a no return policy, even if you think you are getting a great deal. Otherwise, you could be stuck paying too much for that type 1 which you thought was the type 2.
NOTE: I'VE INCLUDED A PHOTO OF THE TYPE 2 MINT MARK, WHICH, DUE TO EBAY CONSTRAINTS, IS NOT A REALLY CLOSE-UP SHOT.