In our area there are to different types of post available, treated and untreated. I think that's common for most of the world. The treated posts typically are using the same chemical preservatives as pressure treated lumber and unlike their very noxious predecessors the modern day pwood preservatives at a lot less harmful to the environment. Natural posts are made from local wood, typically low value softwood such as pine cut to 6 or 7 for lengths and sharpened on one end with a chainsaw.
Here are some of the pros and cons for posts.
Price. $5-7 depending on size. $1.50-2 or make your own.
Life span. 5 years+ depends on moisture. 2-5 years. Will rot faster in moist soil.
Pollution. Some chemical leaching Little to no leaching
Ease. Easy to use. Some variation in size, may split.
Harder to hammer nails. Easy to hammer nails when fresh, harden with age.
One of the main things to remember is that your fence wire will outlast the posts do you'll be doing repairs. The same amount of initial labour goes into both types of fence, it's just a question of spend more money now for treated posts and only do repairs once every 8 years or save money and repair every 2 years.
Making your own posts does allow you to save money, use rot resistant species, or treat your posts by charring them. Charring forms a natural rot resistant layer that can easily double the life of a post and is worth looking into if you have the room. Some people also dip their posts in used oil or tar, but I'm not sure that's a method I want to try in my veggie garden.
Whatever you decide, I hope this little bit of wisdom has been helpful. Happy Wednesday!