If someone is to pay you 100K BTC, it doesn't matter what the exchange rate is, you get 100K either way. Did you mean manipulate down the exchange rate and then lock in the BTC price of a USD-based trade (at the lower exchange rate, thus a higher quantity)?
I'm not sure. What I meant was: push down mtgox-price by selling btc on mtgox (everyone still looks at mtgox for price), then wait for "crowd to accept new price" and buy btc as much as you can on intersango, bitcoin.de, localbitcoins.com, #bitcoin-otc, and any other place that doesn't subsequently run to gox to re-buy what they just sold to you, so without moving mtgox price up again (keep mtgox low by selling more BTC on mtgox if necessary).
After you've done your shopping, let the mtgox market correct to original price (hope it doesn't stick at the lower price)