Pretty new to this game, have a couple of questions you guys should be able to easily answer.

1. What is the mana cap? I keep seeing people trading billions of mana, but I read the cap on selling tokens is 600 million or something? How do you cash out on this much mana? (I've never used a mana token, I'm very confused on this whole process.)

2. How do people get billions of mana? I just got to 200k, and that took a while.

1. You don't recieve more mana from weaves and such after 600m, you can have 2b mana on you at max, mana tokes cap at 1b (0,999b).
What you do is, when you reach say 1b mana, you buy a mana token for that amount, this will create a crystal that is worth the same amount as what you bought it for, this crystal can be traded with other ppl. (The crystal is an item that will apear in your inventory)

2. You get around 30m+ for clearing some weaves on survival, so you can easily reach 1b if your good enough for NMHCMM (Nightmare hardcore mixmode)

Prettymuch what he said, BUT. remember when/ if farming mana like i do ALL DAY LONG
when you reach 600m mana, make a 600m manatoken, or why not at 500M also, when you get 2x the tokens change it to 1b token.

Just so you wont lose any of the mana

So then what's the point of making mana tokens worth 1b if you can only hold 600m at a time?

You can hold 2 Billion mana, the problem is you can only get up to 600m mana when you get it from items you sell to tavern, and from the wave mana's you get from maps.
if someone trades you mana, or you sell an manatoken to bank, or someone buys items from your heroshop, you can go over 600m mana.

A simple answer would be this, Mana tokens allow you to surpass the 600M limit of not gaining mana from the sale of items at the end of waves in survival mode.

You can HOLD 2B at a time, but it isn't advised since the game mechanics aren't made to help someone with that much mana. Mana tokens are the same thing as a Piggy bank essentially, they are for you to squirrel away mana into then, later sell to the tavern, or other players for items you want. They hold the value of the mana you use to create them and there is no service fee for creating them.