I heard coin collecting can get "deep". So much so learn, but also a lot of folks now knowing what they have. Similar with sports memorabilia and sports cards.
Hand made knives would be interesting. A friend in TN makes them out of all sorts of stuff, like old files and stuff. He does everything except the heat treat. I have a few of this blades and they are awesome. Good to have lead on someone who can make a knife to my exact specs.
I think many coin collectors get started by using a metal detector to go "prospecting." They find a few very valuable coins, jewelry, etc., and it reinforces their desire to continue looking for the next big score.
I think it also applies to the professional collectors who buy, sell and trade them. Trading for a rare coin in hopes the value increases over time or happens to be more rare than first thought might result in a million dollar windfall. On the other hand, there could be a new stash of the coins discovered that floods the market and makes yours worth less than it cost you.
If doing this professionally, you are only as successful as the information and experience backing up your activities. if it's just a hobby, and you're more interested in the history of the samples you collect, then you probably aren't worried as much about being scammed. Value is in the eye of the collector.
I've been to a few coin dealers. They often buy direct from the US Mint. I was looking for a proof set for my daughter -- penny, nickel, dime, quarter, half dollar plus the silver dollar commemoratives for her birth year. He was selling them for about double what the mint charged. I found the same sets for less than the mint's original price. The older a proof set is, the more dealers charged. This was only about 2 years after minting.
A "proof" is an uncirculated coin that has been "double-stamped" to ensure the best possible detail and a very shiny surface around the embossings.
Both my daughters have proof sets from their birth years in my safe deposit box. I also gave my daughter a set when her daughter was born. If nothing else, it's a nice keepsake. Best case, maybe the sets increase in value, or it sparks an interest that becomes a hobby.