Here is the context from a novel 'Angels and Demons' by Dan Brown. I know 'sold' is the past tense of sell, but here, I think the word has another meaning.
Vittoria sounded only slightly more hopeful. "I suppose Galileo could have created some sort of mathematical code that went unnoticed by the clergy." "You don't sound sold," Langdon said, moving down the row.
*
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