I've bought & sold a lot of knives and I mainly use EB & ACK to judge the value of a vintage or discontinued knife that I am thinking about buying or selling because they are only the reliable sources of publicly available data for current knife sales.
If the knife is still in production, then dealer/mfgr pricing would also come into play to set a ceiling on the value of the knife.
If I happen to see a sale here on BF, I'll use it as a data point too but sellers here all too often delete their asking price for a knife that they have sold for NO GOOD REASON, IMO, such that doing a search for prior sales yields few or no useable results. So, I don't even bother trying to do that here anymore.
Setting a sales price for a knife that I want to sell based on the data I find usually involves throwing out the highest and lowest #'s and offering the knife somewhere in between depending on how badly I want to sell it.
If you have not set the price correctly, the "market" will let you know it because no one will buy it and no one will even ask you about it, in which case the price is probably too high . . . OR, the knife will sell in a blink of an eye, in which case you have probably set the price too low.
This, of course, presumes that there is "some" level of demand for the knife you want to sell.
If no one wants what you're offering, no matter how desireable YOU think it is, it will not sell no matter how you price it.
I've often seen this happen here on BF which is a very narrow and fickle market.
On the other hand, if the knife priced "right" on EB or ACK, it will ALWAYS sell w/in a reasonable period of time because those sites attract buyers WORLDWIDE & if you can't sell a knife there, you can't sell it anywhere.
I have ALWAYS been able to sell any knife that I wanted to sell here or on EB for a price that I was willing to accept, even if it wasn't equal to or greater than the price that I paid for it.
I've never tried to sell a knife on ACK because of their 25% consignment fee and because I have issues with the consignment sales process, but I have bought several knives there for what I considered a reasonable price.