Both AVE need to be reserved at a train station in Spain. It's not possible to do it online. So do reserve your train back from Marseille to Barcelona already when in Spain.

Thank you so very much. 
Let’s hope there are still available seats as we arrive only a couple of days prior

It should definitely be the case and there is always a plan B:

- AVE Madrid-Atocha - Barcelona-Sants always availble on the day

- regional trains via Portbou

Obviously it will take longer but you get to see more of the Mediterranean. :)

(Still a shame that they're not available online, any news?)

RENFE did promise to make all their content available, after they were investigated by the European Commission over concerns that they may have abused their dominant market position by refusing to provide rival ticketing platforms with full content regarding their real-time service data and their range of tickets, discounts and features:

https://www.railwaygazette.com/renfe-ticket-sales-competition-commitments-made-legally-binding/65731.article

This means, in principle, that it should be possible to book all RENFE trains by third-party ticketing platforms. That should include Eurail/Interrail. I'm not sure what the problem is with the international AVEs though. Rail Europe and Trainline offer these AVEs online, but no pass reservations.

@Mukhammad @Camilo. Will the international AVEs be available via Eurail/Interrail anytime soon?

What a promising news! Thanks @rvdborgt to let us know

As for now, no further information regarding the availability of AVE reservations via our reservation system. I'll ask the product team if there's any development on this, especially after their commitments have been released