Cu placere and N-aveți pentru ce.
Cu placere = with pleasure or your welcome.
you thank someone - Mulțumesc (thanking someone - I thank you) or Mulțumim (thanking the person as a group - we thank you) and they reply....
Cu placere!
Sometimes you might be somewhere a store or place where someone has helped you they might say to you...
N-aveți pentru ce.
Translated directly well that's just too confusing really...something like 'you have nothing for what.' This really is similar to cu placere, means 'no problem.'