I don't have rules next to me to consult, but "No graphic rules" file, which is here, says the following:
Sell wine: You may sell all matured wine cubes of one variety from one of your barrels.
The underlining is as it is in the file.
I remember playing this game that way, so I suppose, this is the correct ruling. To sell the wines from all the barrels would be overpowered in other turns too. And some improvement tiles would be useless, e.g. aging.