how or what would I use to send funds to the seller during checkout ?
Once you've bought Bitcoins on the TradeHill exchange - you *could* use their "Withdraw Funds" link to send directly from TradeHill to the merchant's checkout.
(Withdrawal method BTC)
If you do this - you're effectively using TradeHill as your 'wallet'.
It looks like TradeHill has an estimated time of 'up to 12 hours' for Bitcoin withdrawals... so I don't know how practical it is to use TradeHill as a wallet, because perhaps some merchant systems may time out.
Personally I think MtGox is better.. but you'll get plenty of dissenting opinions around here on which exchange to use I guess.
So - you might also consider downloading the bitcoin client from bitcoin.org, and withdrawing your coins to the wallet on your computer. This has a separate set of security concerns - but ultimately it's probably the best way to be in control of your own bitcoins.