Is a user's Steam Inventory the same across computers?
Solution 1:
Yes. The inventories are tied to the Steam account, not the computer.
However, keep in mind that if you're trying to "share" your account with somebody... well, you shouldn't. It's against Steam's TOS and can create a lot of problems if you're trying to do things on both computers simultaneously.
But if it's just you, and you're using Steam on two different computers (at different times), your Steam inventory is the same on both computers. Get a new gun on one and you can use it later on the other.
Solution 2:
To add on to what @DJ Pirtu said your game library is not always the same across computers. This is because free games only show up in it if they are installed and so on what PC you may have 9 games whereas another 10.
I know this is about steam inventories but libraries are very closely tied onto that.
Solution 3:
Every aspect of your Steam Account is linked to the account and not the computer.
With this said wherever computer you are using, as long as you are logged in, you will have everything that you have in other computers that you used. You will have the same games (of course you need to download them in the different computers), the same saves, (Saved in the SteamCloud. If your local saves and your SteamCloud saves are not the same, Steam will ask you if you want to download the SteamCloud saves to your Local saves.), the same inventory (collection cards, game weapons...), etc...