One of the most important tools available to you when it comes to tracking your online sales, or lack thereof, is the use of the sales rankings provided by some of the most important online retailers. Unfortunately sometimes going through these rankings is a lot like going through a maze. In these systems no two books can have the same ranking at the same time, and they come to a specific order by taking a number of factors into account.
Obviously one of the most relevant factors that contributes to that ranking has to do with the number of copies sold in a period of time. Because of this you must keep in mind that movement doesn't always means sales, sometimes it means lack of sales. Of course, particularly when it comes to those books that have a limited number of sales, assigning one unique ranking to a book is not always as easy as it seems, and the results are not necessary fair. Sometimes when the sale was made can be a determining factor, something like the fact that the more recent the sale, the higher the ranking, but even this is not always possible, so other factors may come into play that are only known to those who programmed the database in the first place. Though spectacular as far as the general numbers are concerned, the impact of these tie-breaker factors is pretty minimal, given that these titles don't usually have the most impressive rankings to begin with, so you really shouldn't worry too much about them.
But let's not get ahead of ourselves here. When a book is first listed it does not have a sales ranking, this is added only after the book sells at least one copy, though depending on the retailer, the absence of a sales ranking doesn't necessarily mean an absence of sales. While Amazon maintains a ranking system that includes every title that has ever sold a single copy, Barnes & Noble only list the rankings of those titles that have sold within a certain period of time.
Now, when you are dealing with very few sales, each copy you actually sell has a very significant impact on your ranking, in fact this impact may sometimes be measured in hundreds of thousands. When you sell a book your ranking jumps up significantly, but after a while your ranking begins to decrease again due to the absence of new sales and then it jumps up again when you sell another copy. What happens here is that, as time passes, your book goes from a book that sold a copy in the last week, to a book that sold a copy in the last month, to a book that sold a copy in the past two months and so on. The longer the time, the lower your sales ranking (or higher, to be accurate. Remember that the smaller the number the better the ranking).
You should also remember that there are other factors that can also modify your ranking. Given that these systems rank you in relation to other books, the performance of those books also plays a part in the final tally at the end of the month. These ranking systems don't tell you how your book did in a vacuum (that would be a system that told you something like this book sold these many copies in this period of time), they tell you how your book did when compared to all the other titles that are part of their database (in fact these systems tell you, in rather impolite terms, that there are these many books that have done better than yours). But lets take a closer look at the ranking systems used by two of the most important online retailers.
When it comes to amazon.com (at least for the time being, this system may be modified at any given time), their ranking is updated regularly, but not all of it is updated at the same rate. The top 10,000 sellers are updated hourly to reflect the changes of the last 24 hours, the next 100,000 are updated daily and finally the rest of the database is updated weekly. This information can come in handy for new authors who often behave like expecting fathers (rigorously going back to check their rankings on a daily basis), because it tells them how often they should really check back. In other words, if you see that your book's ranking is 2,198,429 then that means that you probably shouldn't check back until next week, but if you are close to the top 110,000 then you may want to check daily to see if you've managed to make it into the top 110,000. It is also important to remember that amazon.com assigns a sales ranking to each and every book that has ever sold a single copy, and this means that a number of books with better rankings than yours may actually be out of print.
Barnes & Noble on the other hand has a different approach. It gives a sales ranking based on the book's performance in the past six months (or since its release, depending on the circumstances). This means that the abscence of a sales ranking here does not necessarily mean that the book has sold no copies. This difference in the approaches of amazon.com and Barnes & Noble also accounts for the fact that sales rankings in Barnes & Noble tend to show lower figures (for instance a book may be ranked 438,925 in Barnes & Noble and 2,109,934 in amazon). Given that amazon's ranking includes every book that has ever sold a copy whereas Barnes & Noble includes only those that are considered "current sellers", vastly different sales rankings can reflect a similar volume of sales in both systems.
Now, to put these numbers in perspective, I would guess that if you hope to live off your royalties (modestly and with no promotional expenses at all) you would have to be at least in the top 10,000 and remain there constantly (or have several titles performing at such levels as to allow you to maintain a monthly sales average similar to the one a top 10,000 ranking would provide). As you can imagine your chances of doing this with a POD published title are not too good. On the other hand, an author working with a traditional publisher would need to sell even more than that because traditional publishers pay royalties per copy that are lower than those received by most POD published authors. This means that, regardless of how their books are published, only a handful of writers get to live off their royalties.
You should also remember that even though these sales rankings may be useful, and they cover some of the most important retailers, these are independent ranking systems and they don't cover 100% of your sales. For instance, sales that are made through your site or that of your publisher are invisible to these ranking systems but may have a significant impact on your royalties, especially because usually you get a higher percentage for each direct sale. These ranking systems also ignore direct efforts and all those other online retailers that don't offer a sales ranking system to begin with.
In other words, while undoubtedly an extremely useful resource, these ranking systems should be seen as a guideline, rather than an accurate indicator of total sales. They offer some rather tangible evidence of sales and they do provide you with an approximation of when a particular sale was made, but you should be aware of their limitations and keep their relevance in perspective.