If it is just the information about current value that is needed, not documentation for insurance purposes, the most accurate appraisal is coming from Gunbroker and following auctions for similar items that actually sell, not the asking prices.
There are big league consignment dealers on Gunbroker like Locust Fork, Woodlawn Boys, Bryant Ridge, and others who are turning hundreds of things for people every week and that is the real definition of the market value, what somebody is actually willing to pay, not what some appraiser says you should ask. The second good source for the data is Morphy Auctions and the other big national auction houses, again to see what people actually are willing to pay for similar items.