I don't believe I've ever heard of a site that specializes specifically in the appraisal of custom knives. I know of a few consignment sites (Arizona Customs, Knife Purveyor) that will determine value for the purpose of selling, but they might want to physically examine the knives to inspect their condition first.
Other options are- Buy a one year "Gold" membership here at Bladeforums for $30, and you can ask the vast and knowledgeable community here what your knives might be worth. With a Gold membership you can ask the value of as many knives as you want. To purchase a Gold membership click on the "Paid Subscriptions" button in the banner at the top of the screen.
Or, to try and determine current market value, you could research the knives online yourself. If all you want to know is what they are currently selling for, why pay for an appraisal, shipping, and insurance (and risk loss or theft) if you can see for yourself what they are selling for using Google.
Any chance you could mention what the knives are, and who made them? You don't need to purchase a membership to do that. Posting pictures would also help to determine value (you need to use a photo hosting site like Imgur to post pics here).