Diamond estate rings are the used diamond rings; be it vintage or antique. Even if the ring is just one month old too, it will be listed under estate rings. For instance, you bought a diamond ring last month and you sold it to a jeweler recently. The jeweler will resell the ring labeling it under diamond estate rings.
Another major thing that you must take care is that the jewelers usually limit the term estate to the jewelry pieces that age below 30 years and the rings above that is defined as vintage diamond rings. So, make sure the type of ring that you choose is really an estate ring by enquiring the exact age and previous owners of the ring with the seller.
At times, the term estate can be misleading as well. That is, a used diamond ring that is about 300 years old will never be categorized as diamond estate rings. Instead, diamond merchants usually sell those types of rings as antique diamond rings. This can be way too expensive than an estate as well as vintage diamond rings.