If you create a city that both buys and sells a good, only the buy quantity counts, not the sell quantity. I saw that in game too.
[3.2.0.316-d3fabccb]
#city buys / sells amount (can't be different)
vocal cedar
lost dove
To be frank, the same city buying and selling should throw an error instead, as it makes no sense. So we won't be fixing this and instead we will add a warning to the log if a city both tries to buy and sell the same resource
vocal cedar
But perhaps the Romans had cities that sold (the same product) more expensively and bought them cheaper 😉