From a Catholic perspective, was taking this profit sinful?
Whatever would possess you to ask that question? Of course not.
Since the sellers at the time likely didn’t realize the “true market value” of the properties before selling?
I spent 7 years as a Realtor. People have all sorts of reasons for selling property, and all sorts of reasons for not selling it at what true market value is.
True market value is a price at which a reasonable seller would sell, and a reasonable buyer would buy.
The fact that you felt the property was a good buy does not indicate that they were “cheated”.
I once represented a friend of a friend who had a modest house built in the 1930-1940 era in a very desirable neighborhood. I did a market analysis and came up with a price, and he balked. And when I mean he balked, we are talking tens of thousands of dollars - and he had a degree in accounting. I never did discover where he got such a wild hair, but he was adamant he would not list for what it should have sold for. To make it clear, I was asked by several Realtors why it was listed so low - the assumption being something was wrong with the property and we were not telling the whole story.
If the owners were selling For Sale By Owner, then they decided they would “save some money” by selling it themselves, and they decided what a fair price it was. And I use the same comment about people with out a Realtor or without an attorney: "He who represents himself has a fool for a
(attorney) (Realtor).
Some people are of the opinion that is someone else sells below presumed market, that we - you or I - have a duty to tell them that. As a Realtor, I had a duty; but he sold it below market for personal reasons. That was his business.
The people who put their properties in the market below what you may have though was fair market value and their reasons for doing so. Buying it for what the offer it for is not sinful.
It is entirely a different matter to try to convince someone to sell below market; but that is not what you say happened.