When I purchase Inventory Items for resale, I have to pay 6% purchase tax for every single item.
I have created custom sale tax code and apply to the purchase invoice.
After I finished input the data, My Liability Tax Payable Account Show Negative Balance.
The negative balance is absolutely correct. Since you purchased an item and paid tax you have a negative balance until you make a sale which will be calculated against the current tax balance. As long as your purchases are higher than the sales you will have a negative Tax balance, meaning the Receiver owes you money.