Which carseat base fits with the Royal's carseat?
The car seat base and its availability is very much dependent on what Royal set you get. I mean, there are a few sets in the world that include some type of a Maxi-Cosi car seat, and that needs its own dedicated base.
Mostly, however, the Royal is supplied with a "no-name", color-matched "Carlo" car seat. To that, I'm afraid, no car seat base is compatible - it's a car seat that is not meant to be used with a car seat base. The base is simply not produced. You will need to use the pram's car seat without a base, just using the safety belts.
Ideally, you should be asking the seller whether he can sell you the stroller system with a capsule that is compatible with a car seat base (and don't forget to ask for adapters as well).