Is there a third party site with instant confirm that charges little to nothing and is trustworthy enough to use as a temporary wallet for such large sums?
Yes. I can think of two options in such a scenario.
First you could use localbitcoins.com transaction service at a fee of 1% to set up the transaction. The bitcoin sender would act as a "seller", and the bitcoin receiver would act as a "buyer" (using the product they have to buy bitcoins, instead of using dollars).
The bitcoin sender would send the bitcoins to the localbitcoins.com transaction address ahead of time (they could always just cancel the transaction to get their bitcoins back if the deal falls through). They when you meet and exchange the product, the bitcoin sender would send the release code to localbitcoins.com. localbitcoins.com would then notify the bitcoin receiver that the funds are released, and the would be immediately available to the receiver. No need to wait for confirmations to ensure that the transaction is irreversible.
If you don't want to pay the 1% fee to use localbitcoins.com, you could just contact a trusted member of the bitcointalk.org community to hold the funds in escrow. There are several people here who would be willing to hold escrow in a situation like this and wouldn't charge anything (although they'd appreciate what every you're willing to offer as a gratuity at the successful completion of the transaction.
