Simple question: Do I have to buy tickets ahead of time or can I just show up? Many thanks!
To get from I'Isle sur la Sorgue to Marseilles Airport you travel by train, not buy bus, and you can indeed just buy the ticket at the station when you show up.
Trains run about every hour, and you can look up times and prices here:
https://www.ter.sncf.com/sud-provence-alpes-cote-d-azur/
Not that the station is called "L'Isle-sur-la-Sorgue - Fontaine-de-Vaucluse", quite a handful, and the airport station is "Vitrolles - Aéroport Marseille Provence"