Is there anyplace we can store our luggage for a few hours in Marseille? We would be arriving at the airport and taking the bus into the city. After about 4 hours, we would like to take the train to another city.
I suggest taking the luggage to the train station and leaving it at the storage lockers there: See http://www.sncf.com/en/services/luggage-bicycles-pets.
Rick's Provence & Riviera guidebook does say that Marseille St. Chrarles train station has bag storage (as you would generally expedt in a city of that size), located along track A. The airport bus does stop at the train station.
In fact, the station is the only destination in the city centre that the airport bus reaches, so you'd end up there even if you didn't want to leave luggage.