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Don't trade/buy humble bundle/store stuff from people on steamtrades, most of it is scam and the funniest thing is that you can't do shit, steamrep doesn't give a flying fuck and steam will just copy pasterino you some shit in response. There might be few decent traders, I'm not saying there aren't any but it's better to be safe and not buy that shit from them. If you've got a friend, like a real, decent, existent friend, ask him to buy it for you.
As for the keys, I've been trading for few months now, never had any problem, but then again, never bought anything humble related with them.

They could, but just for scamming issues. By now they're the safest currency in trading, gift games as well.
My recommendation is: Trade with users that have a trusted reputation.

Why did my game disappear from my Steam library?

TF2 / Dota 2 Keys are 100% safe.

Game keys are always a lottery. But TF2/DOTA2 (and CS:GO) keys are as safe as you can get. It would require a VERY special set of circumstances for Steam to remove them from your inventory.

They can be removed if they were stolen from someone's account etc., but if the items you traded for were in the trading window, steam can give you back your items.

Have been using keys for currency for some time, and never had a problem! Iff you are selling to get more games it's the best way

Back in the day, there were reports about people getting in trouble for trading TF2 bought with stolen credit cards. However, for someone who just got like a few keys from trades, there's not really much of a risk involved as the ones facing the repercussions were the people who bought the keys, not the ones who had traded them. Unless, they had taken all of them for nearly nothing (ie. traded a 1 key game for 20 carded keys or something).

It isn't what you trade, but with who you trade the problem. You should do always a background check of the person you are going to trade with.

why are tf2/dota keys safe ?

cuz u trade them within the steam trade window. cant scam anything unless you allowed it. keys are the currency to trade, u can get other things for it - games, items.

TF2/Dota2/CSGO Keys are pretty much the safest trading currency on steam. They rarely get removed and require a special circumstance to happen. The value of the tf2/dota2/csgo keys are also stable. Harder to get scammed other than by value with this currency but you should be ok if you check other similar trades.

The Tf2 Keys are safe, but the humble bundle stuff is unfortunately very often robbed

As far as I know, the only reason TF2 keys would be removed is if their initial purchase was fraudulent (stolen credit card, etc.), and I'm not even 100% sure that would happen. In TF2, when your items are stolen from your account, support will duplicate any missing items and give you those copies WITHOUT removing them from their new homes. The thought behind this is that it's unfair to anyone who unknowingly bought your items from the guy who stole them. People were, predictably, abusing that system though (to a point that people often try to research an unusual's history to make sure it's not a duplicate), so Valve requires more proof now, but to my knowledge, that's still how it works.

As for keys purchased with a stolen credit card, well, I imagine Valve would be less gentle when it's their company being "stolen" from, but that's just speculation.

EDIT: As for Humble Bundles, they're pretty much the worst thing to buy. They have a fixed minimum price of a dollar, so you're likely to either pay a 50-75% markup (1 key) or get scammed (Tip: If someone is offering you something for less than they paid without you having to put any effort into making the sale, it's probably a scam). If you're going to buy Humble Bundles, minimize your chance of loss by buying from someone with no negative rep, and check his rep history to make sure it goes back for a decent period of time. High rep means much less if it's all from the past week. All those +'s may later change to -'s when those bundles are revoked in a month or two.

Anything can be deleted from your account when it's gained through illegitimate means and Valve find out

They will reverse the trade if there is a charge back or stolen credit card, so it is safe. Clicky.