During the 2017 ORCA Convention in Branson, James Appleton of MINICO INSURANCE AGENCY, LLC gave a presentation on collectibles insurance. The audience posed a question about the way to prove collector value of a loss, in the unfortunate event of a claim. James referred back to the insurer they work with, AXA. Here is the response from AXA via MiniCo:

You can use a reputable dealer, ebay sales, auction sales, antique fishing equipment websites with selling prices (there are quite a few) . They can also access the online fisherman for appraisal advice.

Also, in connection with this inquiry, please be aware that Dan Basore has some experience writing letters attesting to the values of collectible items of tackle. Dan sent me an example of one such letter he issued in the past for insurance claim purposes and according to Dan it was very effective in producing the desired results.