Hello new Sinn friends!

I received 2 Sinn watches this week through trades. One was from a new found friend, Jason ("suddha") who sent me a mint 657 with papers/box/cd/bracelet/leather... Anyway, excellent, perfect watch from a great guy.
Watch is a keeper.

I also received a 656 in pretty good shape, although not mint - has a few dings. Nice watch, but not a keeper.

This is my problem: the second watch, which I got a great deal on, came from a guy with excellent creds (on another watch site) and he even gave me time to have the watch checked out before committing. I didn't have it looked at because not only do I trust him, the watch is identical in every way to the perfect watch from Jason, minus the bezel, of course.

My question: now that I want to 'flip' the watch, how do I price it? Obviously I'm not talking exact price, merely how much do I take off as a percentage for the watch being paperless? I know the old "its worth what someone will pay", but that is not what I'm asking.

Lastly, can I send it somewhere to have it 'certified' to be real and in effect get new papers?

Would pictures help?

Thanks in advance! :thanks