Found: Selling to a merchant paid everything in race currency and ignored the currency each item is priced in, so Gold-priced items never paid Gold.
Fix: In the next RF Online 4.55 update, each item sold pays in its own currency: a mixed sale pays race currency and Gold separately, and the sale window shows both. Merchants that sell Gold-priced items now charge Gold for them, so buying and selling stay consistent.
To verify: Sell normal and Gold-priced items in one sale and check that Gold and race currency each rise by the amounts shown.