I am not surprised. When you buy a diamond, plan on owning it forever.
This is very good advice. All the diamonds that I bought, I had "keeping forever" in mind. This is why I have already been careful about not making hasty, impulsive purchases.
Of course, there is always a chance, no matter how slim, that either my heirs or I may decide to sell them. Therefore, I always make sure that the diamonds are top cut, high color no lower than I, high clarity no lower than eye-clean Si1, and graded by AGS or GIA, since top cut, high quality diamonds more likely keep their value and are easier to unload.
A better alternative to selling old diamonds is to repurpose them, so my heirs and I shall enjoy wearing the diamonds without wasting much money and all the efforts in locating and selecting the old diamonds.