Some of my neighbors say such tactics will do more harm than good. We hope so, for you. This is a VERY complex subject. If it affects you, prepare for dull long research. But do not stop at one appraisal if it's low.
Agents usually suggest getting another appraisal before despairing.
A buy down is when the builder pays some of the interest in, say, the first year or two. We have had names reccomended to us but have not been able to locate these people. It's always good to use who friends recommend. But if your friends know a good one, go with that. An assessment is lousy for determining market value.