I commute between London and Paris every week and, although I do not use the ticket resale sites myself, I have three colleagues who do.

Travelling on Eurostar using a ticket in someone else's name is indeed prohibited in practice (who knows about the T&Cs -- but what happens when you try to board the train is what really matters, IMO). If they check your ticket and find it belongs to someone else, the ticket will be confiscated immediately and you will not be permitted to board the train.

That said, ticket names are rarely checked. This is why ticket resale sites are so widely used. For this reason, many people are willing to run the risk of being caught. As an example, out of my three colleagues who use the ticket resale sites (usually once a month for the last year or so), only one of them was ever checked and stopped from boarding the train -- and this only happened once.

I never use the ticket resale sites myself because I value my Carte Blanche status and I like earning Frequent Traveller points for free tickets. But YMMV.