You always end up buying stuff. Mostly because it's easier and less time consuming, and also often because people have a semi-monopoly on the raw materials, often because you can't just build up an entire fabrication facility for a single item.
Take the longsword iteself (helps to have a concrete, real-world example here). It's made of iron. You need iron ore, coal, a good forge (reuseable), the proper quenching liquid (tends to be at least partially destroyed, but semi-reuseable - call it corn oil, for now), the skills to work with it, et cetera.
The iron ore and coal are dug up out of the ground. That's mining, probably Proffession. Of course, first, you need the proper place to mine; not just anyone will let you dig up their land, and usually it's the nobles who own the land, and are willing to rent it to you for some amount of money. Even with permission to dig, nothing says there WILL be ore there. So you need to secure permission from ... whoever. And of course, you need a set of decent mining tools - which includes a light source - a lantern. Or do you have to make those, too? So much easier to buy them.
Guesstimating off of Profession's money-making potential, you get half your check result per week in market price "raw materials". So that 100 gp in iron ore and coal you need, with a +10 modifier, taking 10, take 10 weeks to dig up out of the ground. If there's already a seam handy.
The oils a little tricky - you need to grow the corn, grind it, yada yada yada. Corn has a growing cycle of, what, about three months? Oh, yeah, and you need the land to work, a way to irrigate, seed corn..... oops. Where do we get that?
Of course, you don't need an entire field of corn - you need just enough corn to make one bucketful of corn oil.
Meanwhile, you have to be keeping yourself fed.... which costs money, unless you're feeding yourself somehow....
There's a reason subsistence farmers still buy things...