Need to buy train ticket ahead of time for Paris-Rouen-Paris on a Saturday?

You can always buy tickets. You need to buy ahead if you want to take advantage of non refundable discounts.

Thanks for the information - just hoping the trains wouldn't be full on a Saturday heading back to Paris.

Whether you bought the tickets ahead of time or on the day of your trip, odds are there will be no seat reservations on the train and thus no guarantee of a seat if the train is crowded.

I buy ahead so I don't miss my train while buying tickets; it is convenient to have that taken care of.

Buying in advance is much cheaper, and on that route, even 1-2 days in advance can make a difference. I'm still seeing discounted tickets for today even. And advance tickets are now partly refundable (you get 50% back in that particular case).