Translation

[SirGod]

I am translating a TED conference and I come across this sentence:

Within a year, Rainforest Crunch was on the shelves. It was a great success. We did our first million-dollars-worth of trade by buying on 30 days and selling on 21.That gets your adrenaline going.

I am not sure what the highlighted part is supposed to mean. My guess:

We earned our first $500,000 by buying (things) for 30 days and selling (things) for 21 days.

[susiedqq]

"buying on 30 days" means that you get 30 days to pay for the goods. This is your "costs"

Selling those goods in 21 days means you recouped not only your cost and your profit , but you did it even before you had to pay for your stock (cost of the product)

[SirGod]

Thank you, susiedqq. Now it is all clear.