I have spent a good part of the morning googling and searching the forums for this answer and I am hoping I can have some help to speed it up cause I am coming up short. I am running Out of the Abyss and my players are all remote playing on roll20 using dnd beyond. They are about to acquire a magical item that's in the Abyss book. I wanted to solely purchase that item on this site (I don't own the book on dndbeyond cause I use a hard copy) and then give it to whichever player ends up wanting to wield it as a prize and to make it easier for them to use it.

Can I do this? Or does my player have to essentially 'buy' it for themselves so they can have access to it? 

If you have a Master Tier subscription you can turn on campaign sharing for the group and share anything that you purchase. 

Alternatively, if you own the content here or elsewhere, you can enter the item in as Homebrew yourself, and it will be automatically shared with all of your campaigns.

I recomend using the homebrew as opposed to buying a single item for a campaign. (If it were a class or race, I'd say buy those, but not necessary for items)

Remember you don't need to publish a homebrew creation to share it with your campaign. (Please don't publish copyrighted content)

Agreed with DxJxC.

Creating the item in the homebrew section and using it only within your campaign would be your best bet. 

Alright, thank you kindly for your input everyone.