Im staying in Marseille for two nights next month. I want to take the train to Aix early one morning and return later that afternoon. It is on a Tuesday when there is market going on. Do I need to reserve tickets early for this before I go to France. When I get to France. Or just go and buy a ticket when I'm ready to go and come back. I don't know how much time I will spend in Aix.
It looks like there are a couple of dozen train runs a day between Marsaille-St. Charles and Aix-en-Provence. Most of the Regionales can take you to the Aix-en-Provence station closer to the center of town. There are TGVs that will take you to the Aix-en-Provence TGV station. You probably want the the city center station. Trip takes about 45 minutes. If you take the Regional train, just walk up and buy your ticket. Validate it in the little box in the station platform or at the door leading out to the platform before boarding. Validating the ticket activates it. You can buy your round trip tickets at Marsaille. Just don't validate the return ticket until you are ready to board your return train.
Definitely use the local trains from Marseille to Aix. The TGV station is a good thirty minutes by bus from the city centre.