According to Mark, the 5 types of people you shouldn't negotiate with are:
1. The person who will take your offer to the decision maker.
2. The overly emotional person.
3. The committee.
4. The low-price champion.
5. The “we can’t give a decision as to when we will buy” person.
I have come across four of the five types of people listed above. I wasn't sure who an overly emotional person is. I haven't come across this type of person.
Read his full post here: 5 Types of People You Should NEVER Negotiate With