Many folders maintain their value, even increase in value. Others reach a point at 1/2-2/3 of their original street price and stay there. It really depends on the tastes of the buying public and what is hot at the time. Right now Spydercos and some Benchmades are hot, but some of their older models can be found at lesser prices.
If you buy a new knife, wait 2 years, and try to sell it, there are no guarantees. Unless maybe it is a Spyderco Sprint that is. Otherwise you are likely to lose a little bit but not a lot. If you buy a used knife at a reasonable street price, wait 2 years, and try to sell it, you might be better able to break even.
I've been buying knives for almost 20 years. I've noticed that several that I bought about 15 years ago would sell for about the same price right now. This might not sound bad except that it doesn't account for the increased cost of living since then.