How to buy something from there ?

Keep checking there until you see something for sale. It only restocks maybe....uh....5 times a day or something like that I've heard.

Then there's a button under the item you click to buy. Simple. If you find anything. Oh, and make sure you have Wiz Tokens in your inventory to pay with. If you have to run to buy them or get them out of your vault you will probably miss the item.