To Buy in Advance or Buy at the Station - Tickets from Marseilles to Arles.

rome2rio shows most of the trains on the 40 minute ride to Arles are TER. There is no discount for purchasing TER tickets in advance. See http://www.seat61.com/France-trains.htm#TER

The only hiccup is that many of the french train ticket machines I saw only take chip-and-pin credit cards which are less common in the US than chip-and-signature. I don't know what you'll find at the airport, but if not the machine then there will be a staffed booth.

I did this at Christmas. There was no discount if purchased in advanced. What you'll do is buy a ticket for one of the TERs that goes to Avignon. It's a lovely ride. Arles is fabulous. As is Marseille.

I have a capital one 360 pin and chip debit card. It worked just fine in the machines, but at restaurants they still made me sign.

http://www.marseille-airport.com/
This website for the Marseille airport is often useful. If you go to the Access pages you will see frequent trains from the airport to Arles. You certainly do not want to go into Marseille city, which is in the opposite direction.
These are TER regional trains with no seat reservations. The train stop is a five-minute shuttle bus ride from the front of the airport to the tracks. SNCF insists on calling the stop VITROLLES AEROPORT MARSEILLE PROVENCE, which is what you need to use if looking at the SNCF website http://en.voyages-sncf.com/en/
Here is the page, which is awkward to find: http://results.en.voyages-sncf.com/?clientId=4fe33073-8ecb-422b-a751-99ae5999ec65&language=en&country=EN#/

Since I last used this service I understand staffed ticket booth is now available rather than just machines. Perhaps a more recent customer can confirm this.

I was there six months ago and it was very quick and easy to exit the airport and find the free shuttle to the train station which was a few minutes away, where indeed there was a manned ticket booth.

Thank you! My fear is getting to the train station and there being no tickets left!

Two years ago we arrived at 11:00 Saturday morning and the ticket booth (after we took the shuttle from MRS to the station) was not manned. We were able to purchase tickets from the machine with C & P card and some help from a local there, but it was not without some jet-lagged confusion. Train was perhaps 20% full that day. The tickets are relatively inexpensive, so maybe it's worth it just to be all set, up to you. We got through the airport fairly quickly and found the station shuttle easily.

There's no danger of there being "no tickets left" for a TER train as they don't have reserved seats. If all the seats are full you'll have to stand.

TGV trains do have compulsory seat reservations, but it's very rare for a train to be completely full on departure.