My sister in law has some train tickets which she can no longer use from Poitiers to Lorraine, departing 06/05 at 17h12 and returning 13/05 at 17h13. She has put them on Leboncoin, here is the link if anyone is interested - http://www.leboncoin.fr/billetterie/445 ... tm?ca=20_s
Well that was my brief reading of the tems and conditions on the SNCF website.
I’m not sure if they are promotional prices David, I assume my sister in law would know, she’s French so has an advantage there being able to read the rules! 
Unless they are promotional tickets they are reimbursable at no charge up to the day before departure and on the day but with an admin charge. Seems a bit of a waste of money to sell them unless at face value when you can get your money back from SNCF