alien trade relations

if aliens from another galaxy 

wants to buy a planet in the milky way galaxy 

then those aliens could trade one of the planets in their galaxy 

for a planet in the milky way galaxy 

and/or if aliens from another galaxy 

wants to buy land of a planet in the milky way galaxy 

then those aliens could trade land of one of the planets in their galaxy 

for land of a planet in the milky way galaxy 

it's unlikely that two planets have the same value 

so a valuation is probably necessary 

for a satisfactory deal 

example: 

many low value planets traded for one medium value planet 

or moon, planetoid, asteroid, space station and so on 

depending on how poor those aliens are