not sure. I would say that the Jovians were wiped out by the Darkness, but Xur and the Nine somehow escaped. the Nine want to rebuild their species, but in order to do that they need funds, which comes from them making Xur sell things that Guardians might want. I say "making him" because he frequently says things like "my will is not my own" which clearly implies that the Nine are using him as a sort of "cash cow". once they have what they need, Xur will disappear and the Jovians will rebuild. there is probably something in the Strange Coins that is vital to the Jovian's survival, perhaps their lifeforce. that would explain why Xur wants the coins more than anything else. and you can bet your bottom glimmer he'll leave without selling Gjallarhorn again!