With the right tools, you can get data off of a drive that's been formatted over a few times. For $80 I've bought a program called R-Studio to do data recovery for a few friends after their drives got wiped and you'd be amazed at what was found.
Like Seti, I never sell my flash drives. They are so cheap now that it's not even worth my time to securely delete my data to sell the extras. Hard drives are a different case (and much easier to get data from). In that case, I'd run a program called dirk's boot and nuke (commonly referred to as DBAN) to ensure the data is unrecoverable.