I keep getting rid of things I don't need in my inventory and I've counted and recounted all of my items, yet my inventory still says that I have 30 out of 30 items. I only have 28, but I can't get a blueprint to expand my warehouse because it keeps saying that I have as many items as I can get.

Do you have any items up in the Item Trades or Exchanges? Those take up space until they sell and clear up space/expire and return to showing in your inventory.

I was wondering why my inventory was always full but this still doesn't make sense. When something is put up in trade, how can it disappear from your inventory yet still take up a slot?

It disappears from your inventory so you won't mistakenly use it or think you have it still since the item, or amount of the item, is technically in the trades. It continues to take up a slot because the item is still yours until the trade ends and it goes to someone else. That way, if there are no bids, it can return to your inventory.

Maybe a warning system in case you are about to fill a spot & wouldn't get your item back or a temporary holding system for an hour or so before the item trade dumps your item would be feasable. I just don't like it saying my inventory is full when it technically isn't.

The items that you put in trades are still in your inventory. To get rid of them instantly, you need to destroy them. Otherwise you're still keeping them in your warehouse until a buyer takes them from you and moves them to their warehouse. Auctions don't provide you with the storage place, they just add your item to their list of things available for sale at this moment. Think of it as eBay or Amazon. So technically your goods are in you inventory until you actually sell them.