Hello. New member to the forum here.
I wanted to ask where do (UK based) folks sell their Bartoks when they upgrade? eBay? Forums? Or do people just part-ex.
I have been keeping an eye out for a used Bartok (headphone version) in the usual UK hifi forums for a few weeks now with no luck.
In general most UK dCS owners will not sell on directly if they are upgrading ( the usual situation - it isn’t a brand that once experienced you switch from). They (we) take advantage of the excellent upgrade deals offered by dCS retailers.
So currently I would check with dCS dealers to see if they have any px stock. However the Bartok is a relatively new product and I would not expect too many used items to come to market for some time.
Bartóks are, as Pete has said, a fairly new product, so quite sparse. I am now the proud owner of a Bartók, which took nearly eight weeks to arrive, as they are built to order.
I didn’t demo the unit before purchase, as my dealer didn’t have one available! Normally a no, no, if making an expensive purchase. But, having demoed the Network Bridge, I was more than confident that the Bartók would shine. And my goodness does it shine! 
I can add this: first, you probably have the best chance of finding a Bartok if you put in a request for a s/h one with a dealer. I say this because I and a friend have both upgraded in recent weeks, and I asked our dealer whether our traded-in equipment would appear in their Christmas sale. Nope: already sold to people waiting. The items were never advertised, as far as I know. Second, the distributor in the UK does appear to put out used or ex-dem kit through at least one other independent seller. I don’t really understand why. I’ve stuck with authorised dealers, and have been very happy. Good luck!