This compound word (vender means to sell and humo means smoke) means that someone is selling smoke or thin air, which means that something of little or no value is offered. There isn't an exact translation but in English it would be similar to being a fraudster or chatterbox.
Here are some examples of how to use it:
Las personas vendehúmos tratan de convencer con palabras o argumentos carentes de sentido.
Fraudsters try to persuade you with words or arguments that have little meaning.
Tu amigo es un vendehúmos, no para de decir tonterías inventadas.
Your friend is a chatterbox, he won't stop bullshitting.
Aquel comercial es un vendehúmos, miente para sacar beneficios.
That salesman is a fraudster, he lies to make money.
This word of the day has been contributed by LAE Madrid, the leading Spanish academy in Madrid. Accredited by the Insitituto Cervantes, it offers Spanish courses for all levels and also has Spanish classes for kids and families.