So, over the years, I bought a series of D&D digital manuals/books (in bundles too) on Roll20, and some on D&D Beyond. The vast majority of items I have are digital items on ROll20, I  really need to use on D&D Beyond, but I cannot see any way to link them. I do see how to link from the opposite way (D&D Beyond to Roll20) but not the other way around. I would hate to have to rebuy all the books I already own in Roll20. My goal is to use D&D Beyond with the browser app called AmongVTT.

Please come to my aid, Ye wizards of technology.

:-)

Never heard of AmongVTT, there is AboveVTT, did you mean that?

There is no way to do what you request. There is nothing that will use Roll20 sources into AboveVTT.

Your best option to still use everything is to use Beyond20 to use D&D Beyond things with Roll20 and run your games on Roll20. Even that may still require some work such as downloading maps and uploading into Roll20. You could download maps from Roll20 and upload into D&D Beyond Maps tool or into AboveVTT directly. However, you won't have the non-map stuff for easy use and clicking, like you could by using Beyond20.

Correct. Going off my crappy memory. It should say AboveVTT, and not AmongVTT

 Sad there is no options to use my ROLL20 purchases in D&D Beyond, so it can run AboveVTT on a browser.