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.