Second hand Eurostar tickets

You could try:
https://www.facebook.com/groups/5350926658/

Although Eurostar say:
https://help.eurostar.com/faq/be-en/question/Can-I-change-my-booking

While names, departure and destination cities cannot be changed, if you want to travel on a different date or at a different time, we’ll be happy to exchange your ticket.
If you’re travelling in Standard or Standard Premier, you’ll just need to pay a £30 exchange fee Standard or £40 exchange fee Standard Premier and any difference in fare (per person per leg of journey).

There is extra security at the Eurostar terminals and Passports/ID cards are occasionally getting checked against the ticket details.