low in thus market are very unusual, so they likely have priced it too high. If they kill the deal then they risk the same thing happening with another buyer. If you are doing an FHA loan then that appraisal will stay with the house for six months anyway. That means that any buyer that wants to do an FHA loan has to use your appraisal.
So the burden really is on the seller right now. I personally would be extremely reluctant to buy a home over appraised value in this market.