These cost £500 new so I am not sure what a pair thats pretty much good as new (albeit I have had them for a good few months) go for. I had to get a smaller pair because it was for the living room.
There is a pair being auctioned on Ebay at the moment. Suggest you see how well they do.
I would expect speakers to go for up to 75% of the retail cost generally, and cheapest I can see is £499 on the major dealers websites, so ask for around 375-400 quid.