I'm on day three with Shiki and I went into that store at the beginning. I tried buying some stuff, but I don't seem to be buying anything. When I leave, I don't have what I wanted. I feel stupid for asking, but how do I go about purchasing stuff? And how do I trade in those special red pins?

Oh if you mean your character doesn't change, then don't worry about it. That's just ho it is. Clothes and accessories just change your stats and abilities, not your appearance. When i played through this the first time i was thinking the same thing though.

Once you buy the desired item, go to your inventory and just drag the panel of the outfit to the character you want to equip it to. there isn't much to it really.

quote Rabla

How do you have this pin if your only on Day 3?

huh, that's true. AR maybe?

To buy something, you drag its icon to the shopping cart in the top right.

quote Rabla

Well, the Red Skull cant be traded.

How do you have this pin if your only on Day 3?

He's talking about Scarletite.

Scarletite can be traded in at certain shops in the same way you purchase something, except instead of exchanging money, you exchange items. FYI, most (all?) items you get from trading in Scarletite are stickers, which which usually boost your Partner's or Neku's abilities in some way. These items can only be traded for once per shop.

quote ViperSean

To buy something, you drag its icon to the shopping cart in the top right.

That's what I needed to know. I have bought some stuff now, sorry, meant to reply back to this sooner.

quote Rabla

How do you have this pin if your only on Day 3?

A red pin was given to me at some point, I don't remember if it was handed to me by somebody or if it was a spoil of war. When I examined it, it said that it can be traded at shops for something, idk how to trade it in though. Do I just "sell" it to the garbage can?
And do I ever get new Pins? I have had these same ones since FOREVER and they are all maxed out. I want some new stuff to play around with...

There are numerous pins in the game. You can buy them at shops, get them from competitions (usually just Tin Pins) and also from defeated enemies.

You can find an incomplete list (and info!) here. We are working to complete the entire pin index there though (with useful info unlike other sources).