You also get SR and SSR accessories in the Union Events rewards. You should keep all the SRs that you get. They are useful as fodder for leveling SSRs. You can equip them on himes without leveling them in the meantime. Ns and Rs can be sold or not depending on the way you intend to level SSRs. The rings in the acc point shop have random effects and you are guaranteed 1 SSR accessory when you complete acc quest levels 4 and 5, so they are not worth buying to be equipped. They can serve as fodder.