There are no extra costs involved over and above the purchase price + p&p.
I have found German eBay sellers, both private and business, to be honest and very easy to deal with.
Many German sellers now accept PayPal, but there are still some that expect a transfer direct to their Euro bank account, which can be expensive depending on your own banking facilities. I now only deal with those that accept PayPal, but in the past I have sent Euro banknotes through the post and have not once been cheated.
Most Germans have a reasonable understanding of English, but I always try to correspond in German even though my knowledge of the language is fairly rudimentary. I started by including an English copy of my communications, but don't bother now. Earlier this year I bought one locomotive from a dealer that turned out to have no motor in it, and managed to negotiate a refund of £43 (the cost of a new LGB motor from my local dealer in England). The dealer said that he doesn't test any of the second hand locomotives he sells on but will generally refund what the buyer thinks is reasonable if a part is broken or missing.
Hope this gives you the confidence to try.