Well, not really. As this excellent article in today's Washington Post Book World more than adequately demonstrates, most bestseller lists are exercises in prejudgment as much as anything else. Some in the publishing industry would argue that this is only a reflection of the advance-order system that drives both print runs and warehousing. This is a valid consideration, particularly since the various lists only rank the candidates—they do not disclose how many sales it took to get a given rank in a given reporting period. In other words, some #1 bestsellers are more equal than others… but readers can't tell.
The thing that bestseller lists do not, and cannot, disclose, though, is the single figure that is most relevant to any press smaller than the Big Five: eighteen-month cumulative sell-through. The reasons for such importance are myriad and seemingly inconsistent. "Cumulative sell-through" is the number of copies sold to an end-user, exclusive of remainders, divided by the number of copies shipped to either end-users or the last step before them (e.g., bookstores). That's copies shipped, not copies printed. Here's an example, taken from a real (now resolved) dispute and suitably disguised:
5,000 print run
3,419 shipped returnable
674 shipped direct
722 returns
The sell-through ratio is ((3,419 – 722) + 674) / (3,419 + 674), or 82%. As you might imagine, this was a $70ish specialty-press work, which accounts for the astoundingly high sell-through. Eighteen months out, the publisher has enough data to make an intelligent decision on remaindering the 1,629 books in inventory. As I've discussed before, the off-books remainder process will probably account for the printing cost of those books if all are remaindered, even though accounting practices will never result in a publisher admitting that.
In any event, let's just say that bestseller lists are about as reliable as early exit polls and leave it at that.