There is a rule in newer databases ... "EnableAltCurrencySell" -- haven't tested it.. but I am guessing it works
If you don't have it...
Code:
INSERT INTO `rule_values` (`ruleset_id`, `rule_name`, `rule_value`, `notes`) VALUES (1, 'Merchant:EnableAltCurrencySell', 'true', 'Enables the ability to resell items to alternate currency merchants');