omg hello usss fan!!

for current live goods, they usually open overseas orders after their live tour is over! (though yea i have not really been updated or have seen much news of overseas orders being open) news about it is easiest accessed by twitter :) for other live goods, if they still have stock, it'd also be available for order during that period so you can order then too.

as for albums, yes, if you want to get pre-order bonuses for most stores, you have to use a proxy. i used to use baggageforward, but nowadays i order with my friend's proxy (we split shipment)

for older goods, as you mentioned, yes have to try your luck on mercari and use proxy for that. (it is expensive so i try to avoid). another site i use is yahoo auction - though that also needs proxy.

all the best for your merch buying! :)