I have encountered a couple of problems with traders over the past couple of weeks, one of which may even be a feature rather than a bug.
The first: attempting to send a trader to a hub from a town without the "Faction Markets" technology results in the trader disappearing. The trader does not appear in the town or the hub, and is not counted when determining the wine cost for the next trader.
The second: traders do not require a guild, and there is no way to dismiss them. I trained a trader in one town, sent him to a hub, demolished the merchant's guild, and in hoping to transfer the trader to a different town, returned him to his town of origin, thinking he would leave service much like cotters would. He did not, and so I'm left with a trader in a town with no space for a guild or trade office, making him useless in all but raising the cost of my next trader.
The loss of my trader does not bother me, but it'd be nice to be able to dismiss them at will. Would be wonderful if someone could look into this.