Actually, and unfortunately, there is no quick AND safe way to do this. Let's suppose that the player pillaged wont resist, cant resist, would not attack to take the resources back, and wont receive any help by anyone. Let's assume for that scenario 1 an inactive player AND that no one will attack your town. End of scenario. Freighters could need days to load and travel, but eventually they will take your pillage back home.
Scenario no 2, the player resists, or some else will try to steel your gathered loot. End of scenario.... There is no way to protect the res. The mobile town is small, build up, it bits the purpose. Move army and navy to protect the res by fighting, bits the purpose too.
Scenario 3 occupying another town, will work with scenario 1 facts, indeed you half the time needed, with 10% cost. However, if player defences, or host town v-mode or another player interfere, back to scenario 2.
Conclusion, if undisturbed, use freighters. If disturbed, find another target to pillage.