well, in fact, the port does get them. It just does not sell them back
The rarity is used to get the prices to the rest of the ports. Then at the daily update, we delete them
I do not see a point for a port to sell other resources. It will mean a totally different market system with the trade routes we have now. This system is fully automatic and has many ways that can be utilized - I have tested it extensively in my test account.
At the end of the day, with total cargo of 1 fleet being at max 700 crates, who is going to manually go around ports checking prices and manually trading ? Especially by the time that there is already a more sophisticated system in place that lets you do the exact same thing without the requirement of micromanagement.
If you wonder how can this be accomplished, I suggest you take a look in how you can utilize warehouses. You can stock your resources there till you get a good price