It's true, this was a liquidation sale, but most of the records came from a batch of 8000 records that I bought for $2000.
I was being a bit flippant with the ROI percentage, sorry about that - just trying to stress how easy the process was - I really was blown away when I kept asking people how they found out about the event, and they said they saw an ad for it on Facebook.
One thing to keep in mind, as far as calculating ROI, is that if I were to continue to run the business, these are all new customers that I could continue to sell to, so the return on investment isn't purely a one-time thing - the LTV of each customer is way more than they spend in one day.