One of the things that new collectors are often told is that if they build a set, the collective value of the coins will be greater than the individual value when it is time to sell. Is this correct or is it just clever marketing hype? I believe that the answer to this question is yes, no and maybe. Let's take a random example of a set--Charlotte quarter eagles--and look at instances where there would or would not be a premium factor established upon completion.
There is, in theory, a clear-cut instance of when a set of Charlotte quarter eagles would gain value if it were complete. This would occur if all the coins were very high high grade and the set would be almost impossible to duplicate at any price. But what if the coins themselves are not as impressive as the plastic they reside in? I have seen sets of Charlotte quarter eagles in which all the coins were accorded very high grades by PCGS and NGC but the coins themselves were unimpressive; some were recolored while others were puttied. Among well-informed collectors of Charlotte quarter eagles there are high grade sets that are famous for having great coins and there are sets that are (in)famous for having coins that are "maxed out" and unappealing despite impressive grades.
A set of Charlotte quarter eagles might not have to be high grade to be impressive and to gain value on a completed basis. I have seen sets where all of the coins were "only" in the EF to AU range but the individual coins were gorgeous with matched natural color, nice surfaces and strikes and strong overall eye appeal. In this instance, I think a set could gain as much as 10-15% premium. The reason it would gain value is that a potential buyer would realize that in today's market--where most Charlotte quarter eagles are stripped-n-dipped--the opportunity to acquire high quality coins is rare; and the opportunity to acquire a complete set of them is even more rare.
An instance where a "maybe" answer might have to be given is with a clearly mixed quality set. I know of a few sets of Charlotte quarter eagles where the quality is wildly uneven. There might be a common date in EF45 which isn't very nice alongside a rare date in MS63 that is spectacular. This lends itself to a sort of numismatic version of "which came first, the chicken or the egg?" Would you pay a premium for a set that had some great coins but which you knew that you would be forced to do significant upgrades on others? I think the answer has to be made on a case-by-case basis. If the highlights off the set were enough to offset the low-lights than I think a premium factor would be in order; just maybe not the 10-15% that I mentioned above.
There are other instances where I think that a set premium would be in order. I would pay a healthy premium for a set that all the coins had good pedigrees (not necessarily famous pedigrees but they may have come from good retail dealers or not-so-famous auctions that have a high regard among specialists). I would pay a premium for a set of coins that were original. And I would probably pay a premium for a set of early gold coins in which each piece was better produced than usual.
Here's the rub on set premiums. Most collectors enjoy the thrill of the hunt and putting together a set. But the concept of long-time collecting has becoming somewhat antiquated in the Internet age and today's faster-paced lifestyle has shortened the attention span of everyone; even middle-aged coin collectors. Not everyone has the patience to wait five or ten years to assemble a nice set of Extremely Fine Charlotte quarter eagles. The chance to knock out the whole set with one punch is appealing to certain collectors who can afford to write a one-time check.
In the very high end of the market, opportunity costs have to be considered. If you are a serious collector of Charlotte quarter eagles (or anything else for that matter...) the chance to buy an incredible one-of-a-kind set may truly be a once in a lifetime opportunity. In this instance, the chance to buy a great set for a 10% premium seems to me to be a great value.
Over the years I have purchased a number of superb specialized collections. There has never been a time that I didn't want to sell the collection intact, preferably to a loyal collector who, I hoped, would sell it back to me at a later date. I've even been willing to sell it at a discount price, as an intact set, knowing that there was a chance I could handle it again in the future. But in virtually every case I have had to break the set up and sell it two coins here and three coins there. Why? In the case of million-dollar and higher sets I think price has been as issue. There are not many collectors who can (or would) write a check for a million-plus dollar specialized collection.
As I mentioned above, the "set premium factor" is a term often used by marketers when selling coins to new collectors. This concept is typically negated by the fact that these marketers are selling lower end coins or they are in the midst of promoting material that is common and isn't ever worthy of some sort of premium factor down the road. But I do believe that set premium factors do exist and that savvy collectors can create their own mystique when it comes times to sell.
If you don't believe this, study the results of the recently-concluded Steve Duckor collection of Barber half dollars. Yes, the coins were great. Yes, the selling venue for them was ideal. And, yes, Heritage did a fine job cataloging and presenting them. But I honestly believe that many of the buyers, consciously or not, paid a strong premium factor because they believed (rightfully, in this case...) that "if the coin had a Duckor pedigree, it had to be good." This was a case where there was a set premium factor; in some cases a remarkably strong one.