Online version of my physical game?

If I have already purchased the physical version of a dnd campaign that is in the Roll20 marketplace, how do I transfer that into Roll20 so I can play it online without paying for it twice? Thank you!

You don't.  Marketplace purchases are a separate entity from a physical copy of them.  They represent a different product with shared material, like the difference between a book and a movie—owning the novel doesn't get you into the theater for free.  The Marketplace version is the culmination of hundreds of hours of effort by different people to create the Roll20 experience.  If you aren't interested in paying for the Roll20 produced version, nothing prevents you from putting in your own time to create a game with the content transcribed from the physical version you already own.

You'd do things like take a picture of the Maps & Illustrations, size it for web use, upload it to your personal Roll20 art library, drop it from your library onto the game table. For NPC's, scenarios, box text, events, you'd just read & say it out-loud over the Roll20 Voice & Video webcam feature. For monsters & combats, you'd be able to use Roll20 chat room to do things like Roll 1d20+5 easily --- really, the complicated macros or sheet buttons are not necessary! But if you wanted every monster to have specific buttons (for Claws, or Breath Weapons, or saving throws required, and range, and duration, and so on), you'd need to make each monster/NPC as 1 character sheet, and fill in all of the abilities on that sheet for each. Because the Marketplace version has all of that stuff already done in Text & Graphics & Tokens & Character Sheets filled for you in Roll20 format. But yes you can & I do run modules from Book / Magazine / PDF / Paper / Notes.  I mean I do it for old modules that don't exist in Roll20 yet, I personally wouldn't want to do it for Rime Of The Frostmaiden because the Roll20 marketplace product of that is very beautiful and well-done, a big time saver would be worth the price for convenience and sleekness. 

Ah!  Apologies, I think I misread your initial question.  What Gold says. =D

Eh, I meant to post an addendum to what The Aaron said, not a correction. Both answers are accurate, different ways of looking at it. My answer was sincere, no joke. You can literally do either way on Roll20.  You can buy a Marketplace module because it's like a Movie of the book.  Or you can assemble, recreate, upload to your own personal tabletop, say on mic, type on chat, whatever you want including any module you have on-hand at home. Because Roll20 is a fresh blank gaming tabletop to make your own.