Sell sb/yourself/sth short Verb + object/complement
To underestimate someone, something or oneself
Don't sell yourself short. You sing beautifully.
They're selling Ellie short because of her appearance, but they don't know that she is the best student of her class. Don't judge a book by its cover.
Used in response to a threat suggesting that the threat is a weak one.
To have low respect or regard for someone or something; To evaluate someone or something with a negative attitude
1. To consider something as true, real or available without questioning it
2. To fail to show proper appreciation or gratitude to someone or something; to underestimate the value of someone or something
The verb "sell" should be conjugated according to its tense.