How to make money from it?
Buy low, sell high, like with any other collectable...but timing has to be impeccable (look at what happened with XM-18 speculation

).
Or, some people turn the hobby into an obsession then into a career; knife-making.

Hard to make money at that too, but some folks manage.
My suggestion is just forget about making money, and simple enjoy the knives.
If you like trading them, go for it, but still just enjoy the activity rather than chasing that cash.
A minimum wage job will net you more cash, generally.