I noticed that right now, only the thief archetype is available. Assassin and arcane trickster are missing, despite being in the player handbook.

For the beta, only the rules and options provided in the D&D 5th edition SRD , D&D 5th edition Basic Rules, Elemental Evil Companion are available in Beyond. Once the full platform launches later this summer, the core rule books and others will be available for purchase, and the content within will be available for those that purchase, or have access through a DM's subscription.

The full platform is running but I don't see any guide on which books offer which options.
If I'm looking to open up the Roguish Archetypes, without spending $400 on the Legendary Bundle, what do we need to buy?

It's frustrating to want to buy something and to be unable to find it. I'm making a swashbuckler rogue but have been unable to find any way to add that to my cart.

The books that currently have Roguish Archetypes in them are the 5e Player Handbook (Assassin and Arcane Trickster), The Swords Coast Adventurers Guide (Swashbuckler and Mastermind), and Xanathars Guide to Everything (Scout, Inquisitive, and a reprint of the Swashbuckler and Mastermind). If you want to purchase the subclasses without purchasing the full books you can click on 'Purchase Options' in the marketplace for their respective books and buy the subclasses for I believe 2 dollars each.

Thank you both! I'd been googling and searching in the marketplace and I kept getting the 404 dragon page in the marketplace with any results.

So let me get this straight. One needs a Hero subscription and to purchase the core rule books in order to be able to add arcane trickster or assassin to their character sheets?

Nope, the very minimum you need to do is purchase the Arcane Trickster subclass individually. 

Purchasing the Player's Handbook gives you that subclass, plus all the other content.

A hero tier sub removes ads and gives you unlimited character slots

Ah yes, I see. I clicked on Marketplace then "store" then scrolled down to the Player's Handbook and added Arcane Trickster from there. Now I see the price of the PHB has dropped $2 as a result of making that purchase. Kewl.

I recently got into D&D and using D&D Beyond. I was looking for the Arcane Trickster Roguish archetype. Would purchasing the player's handbook here mean all features, subclass, spells, and archetypes etc unlock that are within the book? Just want to make sure. I can see myself making many micro purchases in the future if I bought them individually.

Yes, purchasing the sourcebook here is how you unlock them for the site's character builder. Micro purchases do count against the cost of the whole book, so if you did buy just the Arcane Trickster background, and the spells bundle, that would cost against the full price of the book if you were to wait and buy the rest later.

thank-you!

Do you have any idea where to find the Rake archetype/if D&D Beyond has it? I've been looking all night and can't seem to find the right sourcebook :(

Quote from mochimo_plays >>

Do you have any idea where to find the Rake archetype/if D&D Beyond has it? I've been looking all night and can't seem to find the right sourcebook :(

 that isn't an archetype that exists.

Are you referring to this? https://www.5esrd.com/classes/fighter/fighter-martial-archetypes/rake-3pp/

That is 3rd Party (ie not published by Wizards of the Coast) content. 3rd Party content is generally not supported officially on D&D Beyond. Though you can use the homebrew tools to create it yourself for your own use. Just don't publish it as you aren't the original creator of it.