When doing business with an ignorant or naive person, are there certain guidelines for setting prices?

I know a carpenter who found some really old coins while remodeling the floors of his customer. I took a look at the coins and they’re worth thousands of dollars. I knew he didn’t know the value of the coins so I offered to buy the coins for just $5. After our transaction, I then sold the coins through an auction for $1500 a piece, minus a 10% deduction for the auction fee.

A few years ago, a similar situation transpired in reverse, where I owned an item that was worth very little and I raised the price by 1000% for the interested party.

The two people I conducted business with were unaware of the item’s value. I would like to know if there are some written or unwritten guidelines for determining how much higher or lower can prices be set for the items being bought or sold.