Really dumb question about item buying

I had 6 slots items and was trying to build a Aghanim's Scepter (I had the +10 strength and +10 agility items out of the four) and wanted to complete it (I had enough gold).

It wouldnt let me due to no slots, why is this, and what would I need to do to build the Aghanim's Scepter in this situation?

The +10 int item comes from the main shop, but the Point Booster comes from the secret shop. If you go to the secret shop and try to buy it, the Point Booster will go into your inventory and the +10 int into your stash.

However since your item slots are full, it can't give you the Point Booster and you get that error message.

Either put one of the items in your stash to create space for the Point Booster, or (ideally) send the Animal Courier to the secret shop instead of going yourself and buy the item you need with the Animal Courier (once it's in range, you can buy them as normal and they'll go straight onto the Animal Courier ). You can then pick up any components in your stash, and bring them all to your hero at once.

How do I put items in my stash? Is it at the base to interact with?

If you buy something that's available at the main shop while your slots are full or you're not at the shop yourself, it will go into your stash automatically. You can also put items directly into your stash when you're at the main shop yourself, by right clicking them and choosing 'move to stash'.

To assemble an item with multiple components, they all have to be in the same place - that can be in your inventory, your stash, or on the Animal Courier. If you have two components and the Animal Courier has the other two, it will automatically assemble the item when the Animal Courier delivers those components to you.

Sounds like you're fully slotted and have half the items you need for Aghanim's Scepter (the Ogre Club (+10STR) and Blade of Alacrity (+10AGI)) in your inventory.

Since you have half the components, you need to buy the other half. If you shift+click an item in the store, it will put all the remaining components in your quick-buy .

If you wanted a Animal Courier to bring the Aghanim's Scepter to you - you'd need it to bring both the Staff of Wizardry (+10INT) and the Point Booster (from the Secret Shop) items to you at the same time (since your inventory is full). Once you meet with your Animal Courier, your items should combine into the Aghanim's Scepter.

If you are at the fountain store (and already had the Point Booster), simply clicking the item should build it. That being said - I have had one-off weirdnesses trying to build items...usually it's the Magic Wand. If it doesn't seem to be working right, I usually drop an item that's not part of the recipe to free up room, wait for the items to consolidate, then pick up my gear. (Obviously not ideal if you're engaging in anything at the time)

Something to keep in mind when buying the individual parts for an Aghanim's Scepter is to buy the Point Booster first rather than last. At 1200 gold, the Point Booster is a super efficient source of HP and Mana. Given that most heroes only have two slots available when they start working on an Aghanim's Scepter (I generally tend to have boots, wand, random cheap item, TP), you are going to want to maximize the effectiveness of your last two slots. Buy a Point Booster first, and then buy the +10 item for your primary stat. That combo gives you the most bang for your buck regardless of the hero type.