Ah, OK then that's not really a dealer, its more like a second hand web site. Not sure what kind of protection Chioce HiFi provide if any, I was looking at a couple of items on there a while back.
Interestingly there's an 861b for sale at £3,500 which seems over priced (no SE transport not digitial i/o), and its in Guildford so near me!
On balance I would say, if it works fine (which it is advertised as doing "checked for operation"), then its a decent deal. And if it doesn't provide you with what you want, then it should be fairly easy to pass it on with little loss. They also say "or nearest offer" so you could make an offer and see what reaction you get. I would enquire as to what kind of remote comes with it - should be the single billet aluminium one that I have, it weighs a tonne
but it might be a cheap plastic version. I would also check what kind of come back you have if it turns out the unit doesn't work properly.
It has the digital i/o which is definately a bonus, it cost me £1000 to have this board fitted by MDC.
EDIT: I just read the Choice hifi blurb and Warranty, which is for 3 months. I'd go for it, seems like a good deal to me.