Don't trade anything in your bag once Friday <t:1767733200:t> hits.
This is because this is when sales are at it's lowest and the week has concluded. Instead:
Buy the three items on Saturday and then consider to sell 20 of the local fluctuation item on Mon/Tue before the price goes higher (Since it may mark as "Sold out".
Then, when the price is at it's highest points on Wednesday and Thursday, make your sales.
For example if the local fluctuation item is A, the other two are B and C
We buy 30A 30B 30C on sat
Optional: sell 20A on Monday and Tuesday to prevent sell out situation. Then buy 30A again on Monday. Now we have 40A 30B 30C = 100 in total to sell Wednesday and Thursday.
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Buying pattern
After Friday <t:1767733200:t> local time, open the Price Bulletin and sort the three goods by base price.
Default Storage + Feng Cheng rentals
Buy 30 of the most expensive item, 30 of the second most expensive item, and 20 of the remaining item.
Guild level 5 skill level 20 + Mansion Upgrade
Allows for 30 of each item totaling to 90 total items per week.
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Guild Techniques
If your guild is level 5, you have the options for Selling Bonus and Market Tax reduction.
This means guildmate and guild NPC trades give you increased commerce coin. (214%-300%)!!
This also means you have a reduced tax rate as well.
Market Bag Capacity can also be increased to hold more items.
If you cap this, you will only need to purchase the largest plot from Feng and be able to carry x3 stacks of 30 of an item.