Dfyjkt

A bloke frequently travelling Brussels-Paris with the Thalys told me there is a second hand market.

I can't find any such thing?
Any idea? Would it be a French thing or a Thalys thing?

I am going from Brussels to south of France.
(I am kind of thinking that there should be some cancellations after recent events).

A quick google turned up http://www.kelbillet.com/. Searching for Brussels-Paris gives a price comparison including train, bus, air, and car sharing. The train option includes tickets being privately resold by other site users ("billets d'occasion"), sometimes at quite attractive prices.

A word of warning: while regular train tickets in France are not nominative, some may have a name on them (and require an ID to be presented on board), or only be valid for specific groups of people (students, elderly, holders of specific discount cards etc). You'll be in trouble if you travel with such a ticket.

Online ticket markets are relatively safe (they don't allow such tickets to be traded), but double-check will never hurt, as realizing your ticket is invalid is much cheaper before you actually board the train. Needless to say, extra caution should be taken when buying tickets from individuals.

Also note that most second-hand tickets are non-refundable, so you'll be at loss if you're late or your travel plans change.

Trocdetrains is the best website for secondhand Thalys tickets. It's only in French, though. Tickets can't be sold for more than the original price paid by the seller.

Thalys officially requires an ID with the same name as on the ticket but, practically, they never check. Upon boarding the train, they check the date and time on your ticket; inside the train, they scan the bar code. They might, but typically don't, notice if you are travelling with a ticket in the name of obviously the opposite sex (e.g., female passenger with a secondhand ticket in a male name).

There are indeed a handful of those websites where people can sell their non-refundable tickets: TrocDesTrains, KelBillet, ZePass, PasseTonBillet and LeGuichet. If you are adventurous you can try to contact people on LeBonCoin (the French version of CraigsList) or maybe even eBay. When I needed a cheap ticket urgently I used to spend my days reloading pages on those websites (some tickets sell fast) but now I check them through another nice website called LeBonTrain at https://www.lebontrain.co which aggregates all of those tickets in on place. Pretty useful. But you'll still have to contact the sellers individually and potentially meet them face to face. Good luck anyway!