Be sold on something
Definition 1 of 1
Pro Tip 1/3Colloquial Usage
The phrase is informal, best suited for casual conversations or personal writing, not formal texts. 
I'm sold on this new diet plan after seeing my friend's results.
Pro Tip 2/3Emotional Connotation
Implies a strong, positive emotional response or enthusiasm towards something. 
He's sold on the idea of touring Europe this summer.
Pro Tip 3/3Skepticism Transformed
Often used when someone has gone from doubtful to fully supportive or convinced of an idea. 
I wasn't sure at first, but now I'm sold on using solar panels.