Yes, you can book your bus seat from Marbella to Sevilla online. Look at the Gomarbella online bus reservations page for the link you need. Online booking is very easy, just put in your passport or other travel identification number and print off your booking ticket and show it to the driver as you get onto the bus. I have never actually been asked to identify myself on any of the journeys I have made.
Of course you can also book and pay for your seat by going to the Marbella or San Pedro bus station, up to 6 days in advance but I recommend you avoid the queues and offhand treatment at the understaffed ticket offices if you can, by booking online
The journey from Marbella bus station to Seville takes three and a half hours by bus with a short stop at Ronda bus station with just enough time to go to the loo or grab a coffee to go from the cafeteria right in front of where the bus stops.
You can also pick up the bus from the San Pedro de Alcantará bus stop, just before it starts the climb through the mountains.
It's a lovely scenic journey winding your way up to Ronda through the Sierra de las Nieves. Some fifty kilometres past Ronda the terrain levels out into flat agricultural land as you approach Seville.
You'll arrive at the Prado de San Sebastian Estación de Autobuses. This is the big bus station which connects Sevilla with cities to the east and south.
For onward travel to Portugal or the Algarve you will change to the Plaza de Armas bus station, a short 15 minute bus ride away.
I hope this helps you, let me know if you need any more info. Have a great