Spirit binding.

[Anova]

Which items you want to sb depends on what is selling. Generally though, spiritbound items will turn in to materia that increases one of the stats on the item. So, a ring that gives +ACC, +DET, and +Crit would turn in to an ACC, DET, or Crit materia. Whereas a ring that gives +CP is likely to make CP materia. Of course, it also has a chance to become an elemental materia, but you will not get a DET materia unless that stat is on the item.

If you are looking for DET and Crit, you will want Zircon and Spinel accessories, and DoW gear.