For such a big and steady trading volume (daily 5 figure amounts), I think you should contact Coinbase and Circle and ask for a higher limit.
http://blog.coinbase.com/post/74631122841/a-change-in-limits-and-denominationsFor example, a level 2 user will have a purchase limit of $50,000/day instead of 50 BTC. Right now that’s equivalent to about 60 BTC, but as the price of bitcoin changes, the BTC equivalent will vary.
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Remember: If you need to buy or sell more than $3,000 USD worth of bitcoin in a day, that is still possible by upgrading to level 2 which has a $50k/day limit.