Payment Proccessors dont sell their coins on the major exchanges, they sell them OTC
Well, BitPay sells a lot through exchanges like Bitstamp and Kraken:
http://www.walletexplorer.com/wallet/BitPay.com
2015-01-12 21:21:37 -500. Kraken.com
2015-01-12 21:12:22 -500. unidentified wallet [44eaace4eb]
2015-01-07 19:14:24 -900. Unidentified wallet [00a5eacf79]
2015-01-06 03:18:47 -300. Kraken.com
2015-01-06 03:18:47 -1000. Unidentified wallet [44eaace4eb]
2015-01-05 16:21:19 -500. Unidentified wallet [44eaace4eb]
2015-01-05 16:21:19 -200. Kraken.com
If you keep scrolling those pages you will see similar payments to Bitstamp (before the hack).
I haven't looked into the Coinbase wallet (available at the same site).
where they could sell at a premium relative to the market price! Have you guys ever seen the listings for high-volume buyers? They would buy thousands at a time and not even flinch.
That [44eaace4eb] wallet may be some private whale, but may also be some part of an exchange's input wallet that the site has not yet identified as such.
If the OTC market price was higher than the exchanges, then why would they sell at the exchanges at all?
For lots of 500 and 1000 BTC, the OTC price cannot be much different than the open market price, since a broker could easily buy or sell that amount in the exchanges and make a lot of money from arbitrage in the OTC market.