Generally to the seller, but you should probably arrange to use a middle.
People will rip you off .. read the threads in te traders lounge, talk to veteran traders in yuor faction and generally do your research. Be craeful and Good luck.
' ' ',
When all its work is done, the lie shall rot; The truth is great, and shall prevail,
When none cares whether it prevail or not. - Coventry Patmore
What Jeb`el Ras said. The Centrepoint Marketplace is simply a user interface to allow characters and factions to sell their goods, so unless Centrepoint Space Station is actually selling something (we do have several listings :)), you will have to arrange payment details with the seller of the listing.
Best would also be to use a middle. In addition to the suggestions Jeb`el gave you, there is also a page with several Centrepoint-reccommended Middlemen/women on the marketplace itself.