"professional appraisal certificate"
No such animal. :o
Appraisals are opinions, and it is very likely you will receive varying opinions about any one particular domain's perceived valuation, at either the End user or Wholesale levels. I know of no "Official Certificate" one can obtain for $15.00 for the appraisal of their domain name(s), and I personally would
never pay to have a domain name appraised in this type of manner (see the Scams mentioned here in this "Discussions" Forum).
I think the *best* way to get an idea of your domain's value is to get as
many opinions as possible from folks who have been around the industry a long time (observations and first-hand experiences) and then complement that with your own research. The "DNA" here can complement all of this, as well.
My guess it that a potential Buyer has requested this of you (again, see the Scams mentioned here in this Forum).
If you'd like to share some of the specifics, perhaps we can help to futher analyze the potential motives behind such a request, or in that absence, help address your needs.
Thanks.