It may be a short-term advantage, but it will leave them with lots and lots of disgruntled customers which is terrible in the long run if you try to start a business.
IMO all of this just goes to show that the BTC ecosystem is not yet mature enough and that you should only play with amounts that are not meaningful to you when lost.
Actually, not really. Correct me if I'm wrong in the assumptions I make on Coinbases' process (I only gathered from comments since I'm not US based and therefore can't use it).
If a buyer buys the 8th, they then lock the price to what it is the 8th. They first deliver the bitcoins after they have received payment, which takes a couple of days (or is ACH instant?).
They will have received the payment on the 13th, given them a 5 day window (or how ever many days it takes for the transfer) for bitcoins to drop in price.
They then buy the bitcoins (which can be done almost instantly when you're on an exchange with funds) and send them to the user.
See, they first buy the bitcoins when they have the payment. If the bitcoins drop in price, they benefit from the locked price, and the customer gets his bitcoins as promised.
SO! no disgruntled customers, and, they aren't doing anything shady. They simply benefit from the time delay in paymet. This of course only works in their favor when btc drops, not when it rises.
The question however, is what if they are inconsistent in their approach?
If Coinbase refunds in terms of USD whenever the market falls, and buys BTC whenever the market rises, then Coinbase is going to make a ton of money in any form of market volatility... and the customer is the one screwed in both cases.
The question we have for Coinbase is: can we trust their policies to be consistent?
IMO all of this just goes to show that the BTC ecosystem is not yet mature enough and that you should only play with amounts that are not meaningful to you when lost.