[MerLock]

I could be wrong but it seems like the previous owner may not have de-authorized the reader so it's still registered under their account. I could also be wrong on this, but I heard that doing a hard reset on the reader de-authorizes it. Can always give that a try and then re-authorizing it with adobe and Sony.

I got this from the 500 wiki but has notes about what is different on the 505 for a hard rest. Note that I've never attempted this before so don't know the details:
Hard Reset

(erases all contents)

* If the device is on, slide the "Power" so that it turns off. Look for a small pinhole on the back panel (not the side, that's the charging indicator LED) and press the reset button on the back using a bent paper clip (do not use anything sharp like a pin).
* Slide the "Power" button to turn it on. While the screen displays "Starting up ...", you will see the light above the "Power" button flash in yellow. Wait for 35 to 40 seconds, then proceed to hold down the "MARK" and "VOL +" buttons.

(NOTE on the PRS505, do not wait those 35~40 seconds, push and hold the "Mark" and "VOL +" buttons immediately -- the boot window is shorter on 505s)

* CONTINUE TO keep these 2 buttons pressed for about 5 to 10 seconds, until the round "PAGE" button makes a quick blue flash.
* CONTINUE to keep these 2 buttons pressed for another 25 seconds, the screen will show a "Reset All" message, and ask to delete all content.
* Press button '5' to clean up content, this could take several minutes if you have many files on the Reader. When the deletion completes, the Reader will turn off.
* Press the "Power" button to turn it on; this could take several minutes if you have many files on the Reader.
* Connect your device to your PC and log-in to the store.
* Go to Account>Manage Devices – you will be asked if you’d like to authorize this device to your account. Please say yes.
* Your device should be ready to read eBooks purchased from your account.