'Bestseller' is a relatively recent term, first recorded in print in 1889 in the Kansas City newspaper The Kansas Times & Star.But this doesn't seem likely to me. Certainly it's easy to find prior examples, although in most of the them the 'seller' is a person, not the object being sold:
Bradley himself was as quick as lightning, and if he liked his horse, and thought any one else liked him too, it must have been something terrific to have turned him from his point, and he was the best seller in his trade, where eloquence is always wanting. In fact he was once thus addressed by an ex-member of the Commons. "I sat in Parliament some time," said he, "for which I paid £10,000, but I never heard any one talk so much to his purpose as you do." [The New Sporting Magazine 6 (1833), 175]But there are examples of the phrase being use with reference to books long before the century's end. For example--
One good publication gells another, and the more we print, the more we sell of each work. Some careful expences will be necessarily and prudently incurred, in this quarter, for advertisements. A regular prospectus printed as a circular will soon appear, and nothing shall be neglected to advance the interests of the company. Queen Mab and Good Sense are the best-selling books which the company has printed; but every copy of every other book will be sure to sell. [Richard Carlisle, 'Joint-Stock Book Company', The Republican 14 (1826), 96]This leads me to believe that the term was in use even earlier, though I can't for the moment track an earlier citation down.