The Times, of course, is the biggee. Other important lists are provided by USA Today, Amazon.com, IndieBound, Publishers Weekly, and Barnes & Noble.
A book about flea removal from pregnant three-legged albino weimaraners could sell exactly one copy and still be the BESTSELLER in its field. There is no law that requires an explanation on the cover or a footnote inside the book.
Anyone can call any book a bestseller (or “best-seller”), and the label may help it to achieve more sales, deserved or not. Keep in mind that even if a book is on a legitimate list, the fact that lots of copies are sold does not necessarily mean it’s a good book, or even that book buyers read what they bought.
There are even fudged bestseller labels that are more the result of marketing than of statistics, such as “summertime bestseller," "top seller," "number 1 seller" or “underground bestseller.”