I too was ripped off by coinbase. Bought 10BTC last Friday at $424 each (instant buy). Was sitting on them til while the price was soaring. Come Wednesday, they cancel my transaction. I would have sold while it was hovering at $850. $4,000 profit gone. Just like that. I contacted support multiple times with no response. Also tried from a different email address and they didn't respond. Now any time I try to purchase they cancel the order. Stating it is a high risk transaction. I have made eight purchases from them since January. Now the last 2 canceled and its like I'm on some blacklist. 
When the BTC price is quickly increasing it is high risk for coinbase. It is not exchange and they have to buy the coins from other exchanges themselves - and transfering money there takes time. They call this high risk transaction
so when your send your ACH payment to them, they don't actually have the funds yet? how do you know this? this is an issue for the client then.. why would i want to pay for something that someone doesn't even have? that's like asking me to prepay for a service that i haven't even yet received.