TER Tickets

First, don't use RailEurope unless you check other sites first because RailEurope often has a markup of ticket prices and may not show all scheduled trains. Check train schedules tickets on www.sncf.com/en or www.trainline.eu. And now for a response to question that you asked, because TER trains cannot sell out and do not have assigned seats, you can use the ticket on any TER for the routing you purchased within a certain number (7, I think) of days.

One thing about TER tickets is that they often have 25% discounts for those 60 and over, and 25 and under. These can only be used at off-peak times where they are available. It never hurts to ask about senior discounts (or youth discounts for that matter).
Rail Europe is no help for these (no money in it for them), but you can check availability at the SNCF websites.

Remember that since TER tickets are not specific to a particular train, they must be validated before use, or you will be fined. There are machines all over the stations on the way to the platforms - watch others and imitate.

If you buy a ticket at the train station, the agent may print it already validated on the assumption you are going to use it right away. Make sure he or she knows if this is not the case.

How does one determine whether a train is "off peak" ?

To SNCF indicates which travel times are off peak in its fare calendar. I have a link to the one for 2016 (http://medias.sncf.com/sncfcom/pdf/calendrier/calendriervoyageur.pdf) but there will be a new one for 2017 though I doubt that the times will change too much for 2017.

Very good. Thanks. :)