Bladeforums has a market place specifically for custom knives. That is likely the best place you'll get the most views and offers. You need to know the approximate value and then entertain offers. Good pictures of the knife and being honest about any flaws, scratches, chips, etc. are a must. I've sold a couple dozen off Bladeforums and only had one returned, but that knife sold with ease to the next one interested.
For most knives, don't expect a full return on the original purchase, but if they're truly quality customs (even moreso from well known makers), you'll be surprised. I have an original Busse Steelheart II from back in the mid 90's and while I paid something like $200 for it, I was offered well over a $1000 (never sold it though :-). Same with some "late" makers...I've got a couple Bud Nealy knives that I've been offered quite a bit for...so it really depends on the design, maker, and quality, and condition.
ROCK6