Hi cdfgolf,
To successfully transfer from your computer to your Reader, both must be authorised with the same Adobe Digital Editions ID. I suspect that if you bought a second hand PRS-505 it may still be authorised with the previous owner's ID. You can check if it is still authorised by selecting 'Settings' from the home menu on the PRS-505, and then selecting 'About'. In the Adobe section is will say whether the device is or is not authorised for Adobe DRM protected content. If it is still authorised I think if you perform a 'hard reset' (instructions on how to do this are on the main support website) then this will clear everything (although beware that this also deletes all the content, which may be OK if you want to load your own stuff anyway).
Make sure after you've done a hard reset that in the 'About' section it does now say that the device is not authorised for Adobe DRM protected content. If you now try a drag and drop of the books you bought from Waterstones from your computer (assuming you are using the Sony Reader Library software), you should get a message asking whether you would like to authorise the Reader. Enter the same Adobe ID and password that you used for your computer and the eBooks should then transfer successfully (you can check which ID you used on your computer - its' usually an e-mail address - by clicking 'Help' at the top of the screen in Reader Library and clicking on 'About Reader Library').
Hope this helps.
