sell someone or something short

sell (someone, something, or oneself) short

To undervalue someone, something, or oneself; to underestimate or underappreciate the good qualities of someone, something, or oneself. Don't sell yourself short—your writing is really excellent! I know you don't like their pitcher, but don't sell him short; he's inarguably one of the best in the league. I think they're selling this short. It's actually an incredibly good deal!
See also: sell, short

sell someone or something short

fail to recognize or state the true value of someone or something.
1998 Times Mr Ashdown may secure his seat in the Cabinet, but he will have bought it by selling liberal principles short.