Why are there items that have like full of 10 sets and it's free and in stock but says this item is not currently available? It also says that a few people already own the item, they're lucky. I want it, how do I get it?

market,forums.......
or wait for restock that will never happend again

It's because most of these items have been given a long while ago and restocked a bunch of times.
The only way to get them is possibly to make a thread in the market section hoping someone will answer.
Don't get your hopes up on a restock thou because it's highly unlikely.

my set