BitPay equivalent as script collection - possible?

[Sukrim]

I'm still wondering if it is feasible to do the following:

Instead of signing up with BitPay, emulate their way of operating by signing up on some exchanges, depositing some USD and BTC on each and use their API to automatically trade.

Once the payment API gets included in the bitcoin clients it should then be possible to accept bitcoin payments like this (e.g. a 100 USD payment with ~100 USD/BTC prices):
1) get current market average, add a few % for slippage - in this case request 1.05 BTC from the client and a payback address.
2) put a market order for 100 USD on the cheapest exchange - fees and etc. included it costs you 0.99876521 BTC
3) mark the payment as confirmed in your system
4) send back the 0.05123479 BTC change to the user and the 0.99876521 BTC to the exchange to top up the exchange account.
5) mabye even send a signed statement to the user that you exchanged at 0.99876521, so he knows which rate was actually used.

You'd only have to pay BTC network + exchange fees and users might have to trust you with a slightly higher amount than they need to pay for a minute or so. The whole script etc. can be open source, all secrets you'd need to enter would be deposit addresses for exchanges + API keys. For merchants selling BTC it could work the other way round as well, though you might need coloured coins or Ripple to automatically track irreversible USD transfers.

There is already a library in the works that combines APIs from several exchanges. Am I missing something or is the missing thing really just some glue between already existing parts?

[Stampbit]

Neat idea, but one advantage bitpay offers is to clear out your account at the end of each day. If you were to wait on an exchanger it would take 3-5 days to see it. Thats 3-5x as much liquidity you have to maintain. And they will guarantee a dollar amount for the sale even if the exchange takes a dive.