I used to sell and buy tickets on ici-londres.com too. That was also a few years ago too.

More recently I have used gumtree but I would be very wary of this site. The person who sold me a return ticket to Paris in Standard for £50 I suspected worked for Eurostar. I picked up the tickets and paid him but there was no price on them. They also had the word "duty" printed on them and I questioned him on that as I was worried they may not work through the automatic barriers (staff duty?). He was evasive and told me they were compensation tickets. But the tickets were fully flexible and I know compensation tickets aren't. The seller actually booked the tickets for my dates of travel and texted me the PNR. He collected the tickets for me.

I didn't have any trouble travelling but I was afraid I might have been asked for ID at check-in. So be wary of staff DUTY tickets.