The same happened with me.
The buyers wanted me to get the domain appraised from a particular domain appraisal company costing about $50 per appraisal.
Later I discovered that it was a scam when they refused to buy the domain even after me giving the appraisal certificate.
No, nobody has asked yet to appraise my domain. Thanks for the warning guys. I understand it's not a very premium domain but its catchy enough
Do you think $100 is a Good/bad/average deal?