UK values are significantly higher than the US,
A mint 3.5E2 Xenotar is worth about £750 so that's a starting point, but value is very dependant on the condition etc. My camera was valued a couple of times most recently last year by a dealer specialising in Rolleiflex's, that was his selling not buying price though,
You'd get the best prices selling privately, look at Ffordes prices as a guide although they tend to be a touch high. If you sell through a dealer then you get more if it's a commision sale otherwise they'll want to pay somewhere between 40-70% of the price they'll sell for.