For health reasons, my sister wanted to sell her house. To spare her any stress, I took care of it for her.
So today, I would decide the same way as with my sister.. If all the documents for the house are available or easy to obtain (current land register excerpt and site plan, for example), then I would sell it without an agent.
However, if an inheritance community were behind the whole story, or if any complex documents or drawings were missing or something similar, I would involve an agent.
I "calculated" the market value as follows.. First, I compared many properties in the surrounding area, then I added premiums for, for example, a younger year of construction than the comparison properties, better equipment, larger living space, and a small surcharge for the "saved" agent commission.
That’s how I listed the property and sold the house at my sister’s desired price.