Ordering resources increases their base cost by 25%, any values after the comma are truncated.
Firewood is possibly the most efficient trade good for selling, given that it's an infinite resource with high value.
Clothing and tools
There is one caveat: Steel tools cost more in production (2 + 5 + 6 = 13), if produced by an uneducated blacksmith.
Food and consumables
Ordering food explicitly does not increase the cost, due to the above mentioned rounding.
Livestock can be purchased from livestock merchants, which is a requirement for pastures.
Seeds are required to grow crops on crop fields. There is no point in buying each seed more than once. Once purchased, the seeds can be used infinitely. The available seeds can also be checked in the town hall.