to sell goods at a price lower than a competitor

A big supermarket can usually undersell a small local store. 大型超市的售价常常可以低于当地的小店铺。

to not give something or someone, especially yourself, the praise that is deserved

Don't undersell yourself - you have a lot to offer a company. 不要妄自菲薄——你有很多东西可以贡献公司。