Caveat emptor

"Caveat emptor" was a Latin phrase said by the Tenth Doctor after Lucius Caecilius Iucundus informed him that he'd "only bought [ the TARDIS] today." Caecilius perceived the phrase to be Celtic. (TV: The Fires of Pompeii)

Behind the scenes
Thought not elaborated upon in Fires, the concept of "caveat emptor" is that the buyer alone is responsible for what he buys, and has agreed to it in the process of the trade. Literally meaning "Let the buyer beware", the principal places all blame on the consumer for all deals and trades they've made. The Doctor uses this as an excuse to get ahold of his TARDIS.