How do you get them?

I know I get them through advancing (just 1 per "level") or you can buy them or get them by reffering. But I want to get them through gameplay.
I have been trying to sell resources but no one buys them.

Selling resources/units through Bestiary is the most popular way to get Crystals without donating. Maybe you are trying to sell them for a higher cost than the others?

I am not, I use the same price as everyone. I noticed that if I try to sell for more or less, tax gets much higher.

Maybe my bestiary isn't popular since I have been looking at other auctions and they don't get sold either. Or maybe people prefer to buy resources from alliance members.

Here is a sceenshot of bestiary I'm using (no other in range):
http://shrani.si/f/3T/Tg/1gKr3mnK/capture.png

That's possible. It's free market out there, administration doesn't force anyone to like or dislike certain offers. People do it themselves.

how can I buy this items?

See http://xcraft.net/dark.php

How do I donate crystals?