Hi Dave - just another bored evening passing the time browsing in Germany
No real advise that you wouldnt use for Ebay normally.
Try and check feedback ratings.
See if price is actually fair (against new / other suppliers).
Make sure postage charges are not excessive.
My transactions with Germany have been smoother than in the UK really.
I just run the ad through - Babelfish.
Ensure they are willing to post to the UK and pay by Paypal (makes things a little easier).
Sometimes I've been copped for import VAT but usually not.
If its a private seller, one nice trick is to ask them to write on the item details for the parcel "Replacement part under warrenty" - then no additional import charges are incurred (in my experience anyways).
Good luck - Frank