Others have answered fairly well already but essentially its going to come down to the specific knife we are talking about, the availability, the condition, and ultimately/most importantly; the consumer. In general customs are going to be able to be sold at or over their selling price because they are hard to get or the maker only makes a few blades in that style each year. Production knives are more difficult. For instance, I have been looking to pick up a TRM Atom for some time but I still balk at prices well in excess of the original price since they are a production knife that is still being made and I know that there will be more coming out soon.
I disagree.
Most custom knives sell at a substantial loss. Only those custom knives from makers who are extremely popular may carry a premium - and the biggest premium will be from longtime makers who have always been popular, i.e. Loveless, or for those makers who have hit it big on social media and are making the knife of the moment for the Instagram crowd, usually some combination of overbuilt titanium framelock flippers with Damasteel, M390, fat carbon, zirconium, and timascus everything.
Some of those are really hard to explain in terms of where the popularity comes from. The Tracker Dan Bloodshark is a great design, but it's still a cord-wrapped, finely finished prison shank, so I don't see how the secondary market can justify asking $1,500 for one, but they do, easily, while less popular makers making similar designs to similar quality standards out of similar or better materials might not get $250.
Some makers, like Phil Patton or Robert Erickson, make outstanding quality work at a very reasonable or even undervalued price.
But unless you're the Instagram flavor du jour or a famous knifemaking icon, the odds are you'll take a loss selling customs. Even recognized ABS M.S. knifemakers' work can be hard to sell and make any money off it, just because the market for $2,000+ M.S.-made bowie knives is fairly small.
The fastest-selling knives are reasonably priced production knives. Very popular high-end production knives where demand exceeds supply, i.e. Koenig, Shirogorov, TRM, etc. will often sell rapidly at retail + shipping + fees + 10% or so. Things like standard production Hinderers and CRKs where dealers are out of stock often but also regularly getting new knives will sell rapidly at 75% of retail. Rule of thumb? If when they're released they sell out in less than three minutes and the knife community is awash with the tears of people who missed the release, it'll sell above retail.
Obviously you can't sell production or custom knives above the price you can buy them for from the dealers when they're still available at the dealers. There was a fellow selling a tomahawk on the forums here that would ordinarily be very popular and should sell fast... except that model was just shipped to the dealers by the manufacturer, and I happen to know at least two dealers that still have them in stock at retail, so the seller here will likely never see his $125 premium.
For the most part, the best way to make a small fortune off your knife hobby is to start with a large fortune. It is very difficult to make money selling collected knives, so the best thing to do is to look at it as supplemental income for buying new knives if you happen to manage to sell at a profit, because odds are good for every knife you sell at a profit, you'll end up selling two at a loss. So just try to break even on the whole. Especially when you take into consideration that just to break even on a knife you
have to ask for retail + shipping + fees.