I'm sure they are just hiding the fees in the exchange rate. Which could be much more than 1% or whatever they were charging.
Did they check the exchange rate for buying on Coinbase or selling. Coinbase might be playing with the exchange rate too.
If they had any reasonable volume wouldn't it make more sense to stagger their selling? On a long enough timeline it'd probably even out, especially if they did something more clever than "sell X% every Y minutes"
Hell, I'd do it at $6k/month. At which point, they may actually make more that way.