I am referring to the idiomatic expression “I don’t buy it” meaning I don’t think it is true.
The earliest relevant example of similar usage I ran across on google books was this tidbit from Buds and Blossoms of Piety, With Some Fruit of the Spirit of Love: and Directions to the Divine Wisdom, The Fourth Edition, by Benjamin Antrobus (an early Quaker who preached and was persecuted and imprisoned in London), published in London in the year MDCCXLIII (1743).
This couplet is said to be from a letter to the author while in prison, from someone known as W.L.:
Let no Dove-sellers in the Temple dwell,
There's Room to buy the Truth, but not to sell.
Talk English alludes to the implied contractual or transactional context to be found in the phrase, I don't buy it:
if you "don't buy it," then you are not agreeing.
As in business transactions a sale isn't finalized until both parties agree on the price or terms, likewise a truth or fact presented isn't 'bought' (or sold, either) unless the audience and presenter are in agreement.
Corroborating the transactional context of the term "buy", are the many references to the act of reaching agreement on various deals for merchandise or favors to be found within the volumes of Cobbett's Complete Collection of State Trials and Proceedings for High Treason: And Other Crimes and Misdemeanors from the Earliest Period to the Present Time ... from the Ninth Year of the Reign of King Henry, the Second, A.D.1163, to ... [George IV, A.D.1820] (also easily referenced on Google Books). For example (in volume 19, dated 1816):
"Let us sell him the [tobacco] box, may-be he will [agree to] buy it4."
Also, in Samuel Johnson's A Dictionary of the English Language, published in 1773, one of the several meanings for agree:
- to settle a price between buyer and seller.
In conclusion, "I don't buy it" means:
...and a reasonable form of the same idiom (but with opposite meaning, to agree -- that is, "to buy", with respect to Truth) may be traced as far back as 1743, to Benjamin Antrobus.