I have created the daylight ring for vampire. The ring will now appear on Vampire whenever they get the daylight ring moodlet. Now I would like to turn that ring into an item that can be bought from the buy mode or life time reward.
How do I get started with this?

Hrm I think it's possible to do so from a point where it's in a sim's inventory, but I'm not quite sure if upon buying the item it would be possible. I know it's not tooo big of a help, but the teddy bear code can help in regards to how you can apply buffs when it's in a sim's inventory

In the case of the item needing to be wearable, I think you could either check out the backpack code (I think cloning it as well is a good idea since it seems to be slotted to some bone) and see how it works. or, of course, check how Arsil did this mod: https://www.modthesims.info/d/594231/mental-powers.html

I know it's not too much help, but I do hope it helps a little!

RIght now I just want to know how to turn a CAS item into a Buy mode item. The ring that the Sim will wear and the buy mode ring will be different objects. How I can create a buy mode item that looks exactly like the amAccessoryRing CAS item.

Thank you for your help I will look into that mod. But I think it will be simpler if I can just make a Buy mode item and apply a new script to that object. The buy mode item has to look like the CAS ring.