i would try to get the buyer to pay for the appraisal.. also, if their first email to you went something like this one below then its likely a scam. --
"Do you sell a developed site or domain only? I prefer domain names.
Do you have an independent valuation from industry experts (appraisal)? I need an appraisal certificate for domain priced in your range. I consider your name as an investment. In other words I'm going to sell your name later and make a profit. If I overpay I won't be able to make a profit in the future. It's very important for you and me to know the current market value of your domain.
Of course, we must be sure that you are engaging a reputable appraisal company. Please note that I don't trust free or cheap appraisals.
To avoid mistakes I asked domain experts about reputable appraisal companies in a forum
http://domaintalk.ourplace.com/Archive/38042.htm
Juck check this posting.
After I verify an appraisal from you we'll continue our negotiations.
How do you prefer to get paid:
www.PayPal.com,
www.escrow.com, check or wire?
Hope we can come to an agreement fast.
"
Sorry to be negative and I hope your buyer is legit !

sadly mine was only trying to get me to pay for appraisals from one of those sites - which he probably has an interest in.