Honestly, second hand diamond and other gemstone jewelry seems like such a missed opportunity. I feel like people would go for it, with the new focus on sustainability and what-not. I wonder why that's not a thing?
Edit: I checked and multi diamond cluster rings look way more impressive and make single diamond rings look cheap and icky.
Similar to not buying clothing from someone deceased or a wedding dress from someone divorced.
Can argue that this is not very rational. But much about the wedding process isn't :)
That's why your shop should only sell the weddings rings from people who had a long and successful marriage and died peacefully in their sleep at age 95.
A 60 year marriage, ended by death of one spouse shouldn't be unlucky. Especially if it was actually the death of both spouses in some tragically romantical way.
Don't underestimate the power of superstition.