I have sold and bought numerous custom knives on the BF secondary market. Except for a Hinderer XM-18, i have always lost money, usually about 30%. I see this as rent for the pleasure of ownership for however long i had owned the knife.
If i put a knife up for sale it is because i need the money, usually to pay household bills. Pricing a knife realistically helps it sell as most people shopping on the secondary market are there because they are looking for a 'deal' (lower price than original).
I have picked up some very fine customs on ebay for prices i think are on the low side (e.g. Joel Chamblin, Gray Taylor, Ken Erickson). I have also made offers to knife dealers that were accepted at ~30% below the listed price.
There are many excellent full time custom knife makers but only a relatively small number of people willing to spend a large amount of money on a knife. So, as i see it, supply exceeds demand which means taking a loss when selling on the secondary market.
kj