Bit the bullet and got a month of premium. How do I make the most of it, particularly making money on the shop?
Primarily you'll make money by selling high value affixes on weapons or units. Things like Modulator, TokyoTrainBoss' soul, attack3's. If you get weapons or units with a high number of slots, check whats on them and sell it. Check what the market value of the things you have are and sell at that price, people will buy it. Selling rare weapons isn't as profitable as it used to be but you may happen to have some of the few expensive weapons collecting dust in your storage. Don't undercut the lowest price by too much unless you're absolutely desperate. The more you can sell the better.
If you have a lot of time to waste, you can sit at the visiphones all day and buy and resell things people put up for too low a price. Occasionally you'll see things like costumes for 1 10's place too little or some affix that is way below market value. Some of the best times to do this are in the hours after a new AC scratch is released.
If you're feeling crazy lucky you can make high quality units and sell them at high prices. But i would suggest against this as it is more likely to kill you inside.
Aside from that make as many 10, 11, and 12 star passes as you think you'll need. Definitely try and get some 11star unit passes. Take advantage of the free crafting slots if you want to level up your crafting. Use the premium medical drinks that guarantee RDR boost when going on EQs.
