Verified by whom ? There is no official board of domain brokers.
I don't think you should be paying at all for an appraisal.
2000 is a lot but why not submit them in small batches in the appraisals section.
Or a small selection of what you think are your best domains.
Anybody who takes money for the 'service' will always tell you what you want to hear, but will bring no sale.
Also, I am trying to figure out why you need them appraised. Pricing domains is not the problem. You need a buyer first. So you could just wait for offers and take it from there.