OP:
IMO, Blade Forums is the best place to start trying to sell your knives because there are no fees (other than the cost of membership) and there is a large and well established audience of knife lovers here.
On the other hand, people on BF can be really fickle about what knives they are interested in. So, you may get phenomenal interest in certain knives and absolutely no interest in others. I also do not see most people here as wanting to spend A LOT of $ on knives.
A knife up to $300-400 should sell on BF if there is any interest at all, but beyond that it may have trouble selling whether anyone is interested or not. The key to this is all of the "likes" you see for expensive and interesting knives listed for sale that, of course, never sell.
eBay will give you the greatest exposure (largest potential market).
I've sold things on eBay when I had to but between eBay and PayPal you'll drop about 10-20% of the price off of the top w/their various fees and other charges. Some very valuable things are sold on eBay but, as I recall, eBay up's it's take the higher the value of the item sold as well.
The "value" of eBay's involvement as an intermediary in the transaction can also be mixed (the same applies to PayPal), which in the event of a dispute could result in the withholding of any sale proceeds.
Nonethless, eBay is a great place to sell things of little to great value if you cannot sell them anywhere else.
Arizona Custom Knives is another well known place to try to sell knives on but you have to "consign" (send) your knife to them and ACK takes 25% cut off of the top as a consignment fee (plus other expenses).
ACK will not accept any knife valued under $200 and is oriented towards selling more expensive knives. So, ACK is probably a place to try to sell your most expensive knives that you haven't been able to sell elsewhere.
It take some time (they say about a month) to list the knife on their site and you will not receive the net proceeds after the sale until the 2nd Tues of the month following the sale. So, you will not immediately receive the sale proceeds from ACK as you would on BF or eBay.
There are a variety of other consignment and auction sites available where you could try to sell your knives but I think the 3 mentioned are the best place to start.
Good luck!