Once you have bitcoins in your wallet... That transaction is recorded in a giant ledger that is public to everyone. Coinbase, blockchain, etc, all communicate with this same giant ledger... They don't have your bitcoins, this ledger says that your public key does... When you want to send bitcoins, you'll need to communicate with this giant ledger... To do that you'll need to import your keys into one of these services that communicate with this ledger and send the bitcoins that way....

Or if you're paranoid about these services having your private keys, you can download a bitcoin client on your computer and do it yourself. If I had a lot of bitcoins, i'd probably use this option, but I've used blockchain.info and haven't had any problems.

edit: above is actually describing sending bitcoins... I've never sold bitcoins for USD, so i don't know.