I don't like the sound of this.. The middle of this sounds bad
The authenticator needs a token (or whatever) that you -yourself- have to give it. So you can only steal yourselfs credentials.
As an example, there's a steamcommunity bot that trades festival cards. It's probably setup so it only accepts 1:1 trades (so one user can't empty the bot's inventory). From a user's perspective, you choose the cards that you are missing and give back the cards that are duplicates; then you need to use your phone to permit the trade. The bot automates the other side; no need to verify the cards or the trade by phone each transaction, from the owner.
I don't think it's possible to steal one's inventory through this unless, of course, a user made a bot himself for auto-trading and screwed up the configurations allowing 1:all or none:all trades.
It would be probably easier to fool a user into a steam look-alike sign-in page and get their user/pass and then backstab them when they aren't expecting (while vanishing with their inventory).
Your missing an important factor, this mobile authenticator runs on a windows desktop...at least the one I linked does.
I know. It's sus, but as usual some people either don't want to give steam their phone number or don't have phones that can run the steam authenticator. So there are some programs that implement that protocol. I have no idea what limitations and problems arise from their use.
It's also needed for automation, otherwise in
n trades you'd have to verify
n transactions by hand by both parties.
My point being that these programs were probably made by users with legit intentions (trading, farming, collectors) but, as always, can probably be used indirectly for scamming and/or taking advantage of unaware users.
Story of my life.. Until bf starts "threatening" me xDDD
That's a reality TV show I'd watch.

So many questions:
1. Is he high energy and driven like you, or is he the opposite of you?
2. Was he the one that bought the yellow candles for your bday cake?

3. Has he ever won an argument with you?
I hope me being

y doesn't end up what burns the Lair to the ground.
