Ripping someone off? 🤔

Husband and I have a used car dealership (licensed, insured, bonded) and buy cars from different sources. Our biggest “scores” are usually from Craigslist that we buy from individuals. We sell most of our cars through eBay.

I was telling a story about a car we bought recently that a woman listed for a few thousand dollars (we paid her what she was asking). We ended up just detailing it and listing it on eBay. It was a rare, older BMW with very low miles. It sold for over 4 times what we bought it for (we didn’t expect it to do that much).

When I was telling the story, one of our family members got upset and basically told us that we had ripped the woman off because our profit was so high.

So my question is...When people buy things from others, do they have some kind of moral obligation to let them know if they have it listed well below market value? Do you think it’s different if it was an individual buying it for their own personal use and not to sell for profit?

For example, if someone sees a baseball card at a yard sale that they know is worth thousands and the seller sells it to them in a box of stuff for like $5, is that unethical?

I guess my question is...is it up to others to tell people what their time and objects are worth, like giving someone a raise even if they didn’t ask or offering more than asking price for something because they undervalued it? If they don’t, is that wrong? Or is it up to an individual to keep themselves informed and ask for what they should be getting?