The way I see it, if you are let's say a Fire elemental, and you want Rage Potions and a Fire Buckler for your deck, you can do a number of things:
- find a player in your neighborhood who is willing to sell those
- find a player who is willing to craft those for a price
- buy it in an auction
- go to that element's territory and learn the skill
- look for a quest that might reward you with these
- look for a player you might loot these from

If you want a windmill, you can:
-find a player from your team who can build it
-find a player from an allied team who can build it
-find a player neutral to you, who... can build it!
-learn the skill
-capture some random windmill you encountered on your path (just defeat that Don Kichote knight who defends it with an Entropy/Time/Earth deck)
Generally, if one out of 5 or even one out of 10 players in your team knows how to craft these items, the second option is probably one of the most cost efficient. Assuming that 1 out of 10 Fire elementals plans on developing some Water skills, while some other chooses Earth skills as secondary isn't a bad assumption in my opinion. People have different preferences. Some people might even take all the crafting skills, since they are Tier 1, and these people might work as suppliers for other team members, you bring them gold and cards, they give you crafted items in return.
One thing I like about this whole system though is that in case your element is at war with eg. Earth, you can't buy shields from Earth and thus the prices rise, especially while during the war some shields are lost. Other elements might sell you those for an increased price for example.
On the other hand, someone might make an alliance with let's say Gravity, simply to ensure an easy access to siege weapons. You might want alliance with Earth to make mines or with Air to make windmills. This encourages making important strategic decisions like "We can do without weapons for a while, we attack Fire, we have a big supply of alchemy cards, we attack Water, we can't afford to lose our access to Pendulums, we don't attack Time, we need to strengthen our economy, we make an alliance with Air, but after building some windmills we betray them...
If every element had equal access to every skill and every game mechanic, it would be a boring "We attack team A since we don't like them, we attack team B, since they are wealthy, we don't attack team C since they are well armed". The problem is that elements become too similar, and there is little economic and strategic difference between fighting different elements. Of course, the PvP part is still different, but I'd say it is great when elements differ in economy too.