Can I sell a service using SPIP? Can I use SPIP for my professional activities?

You can do just whatever you like...

More seriously, some users have asked us if “we are not annoyed” that they are using SPIP for a professional activity (for example, selling websites made with SPIP, or marketing SPIP training sessions...). The answer is simple: “we don’t have any problem with it”. On the contrary, if we have chosen to distribute SPIP under the GPL licence, it is so that it can be used in this way. If SPIP helps you economically, then we shall be very pleased.