Ce serait manquer d’usage = That would be a breach of good manners.
Il ne manquait plus que cela! = That crowns all! That is the last straw!
C’est un marchand de soupe = He is a regular Squeers.
[This is said of a private schoolmaster who, far from regarding his profession as an honourable one, follows it solely with a view to profit, by having few and inferior assistants and by feeding his pupils cheaply and badly (thus making a profit on the soup). He looks upon teaching as the least important part of his work. Of course, this race of men is now entirely extinct.]
Marché
Par dessus le marché = Into the bargain; Over and above.
Il m’a mis le marché à la main = He told me I could take it or leave it; He made me decide one way or the other.
Est-ce marché fait? = Is it a bargain?