I have used Choice hifi to buy and sell a few years ago.
The prices on the website seem expensive, but what happens is, you sign up to sell your equipment, fill in the details and your for sale price.
Choice hifi take a commission for the sale. I think its 10%. So when they make your advert 'Live', their 10% has been added to your sale price.
Some equipment is shop owned, and some is owned by members of the public.
You can call or email if you wish to make an offer on equipment. Choice hifi will then contact the Vendor to see if they will accept the offer.