GL Accounts in Flex follow a hierarchy to determine which is used during the export.
On Accounting Integration—>Accounts, the various Default settings at the top of the screen are the "bottom level" of the hierarchy (i.e., the lowest level and first to be overridden).
Next level is the Resource Type Accounts section in Accounting Integration-->Accounts. Changes here will directly affect Resource Type settings and vice versa. You can find Resource Types settings by going to System Settings-->Resource Types and double-clicking a Resource Type to open its settings
Then, each Inventory Group has a Sales Account and Purchase Account to refine the criteria further. These are accessed and edited by right-clicking an Inventory Group, clicking Inventory Group Manager, then Edit Inventory Group.
Finally, the highest level is for an individual Inventory Item or Service. To access, open an Inventory Item or Service and go to its GL Accounts tab.