In my experience 3 types of knives will make you money -
1. Knives from a new custom maker, who suddenly becomes popular - but you have to get in on his lists BEFORE he becomes popular, otherwise he will already have raised his prices to the market rate, or it will be impossible to get on his lists.
2. Regular production knives which become cult classics after their discontinuation - but difficult to predict as companies generally don't discontinue successful knives, and will bring them back into production if they think they can make money. Sometimes you get good close-out deals on well-made spyderco knives that get love from the afficionados, but don't break through to the mass market. These can be good buys, but you're unlikely to double your money.
3. Limited editions that are very popular - i.e. LE ZTs and popular Spyderco models in exotic supersteels. Trouble is these are very difficult to get at normal retail price. As an investment, you are not likely to make anything if you factor in the time spent perusing the forums or jumping through the necessary hoops to get your hands on one. Also value is uncertain as a long-term investment because you don't know what else will be produced that might steal the attention and interest from what you have. Those who buy these knives just to flip them in the short-term are generally not well looked upon by genuine enthusiasts.
As others have said, it's nice to buy knives you like and be able to sell them on for around what you paid for them if you want to change you collection, but don't expect to be able to make real money by buying knives.
