Does anyone know a base (pob or npc) which both buys and sells Xeno relics?
Cheers.
There are no NPC bases selling Xeno Relics, they are a mineable commodity. You can mine it on Planet Knossos in Omicron Xi (
here) on a Corsair or Outcast ID ships.
There should be some POBs around Knossos that sell Xeno Relics.
There's a PoB about 40k below the Xeno Relic mining zone in Knossos, that's probably the best bet