Yes, this is a scam.
People are commonly targetted on Sedo with offers to buy a domain on condition that an appraisal is done. Normally you'll get directed to a site that purports to show a number of companies offering appraisal services. The one which the scammer is associated is always the cheapest. You pay for a pointless appraisal and they never come good on the offer.
It can sometimes be difficult to assess how much a domain name is worth, read around the forums and see
http://www.dnjournal.com to get a better idea.
Rgds
Accelerator