Here is a small guide how to keep your items and valuables safe:

- Do not download anything sent to you by a stranger. If they claim it's well known software (such as Mumble or Teamspeak), then use Google or another search engine to find out the official site for that software and download it from there.
- Check that the spelling of any familiar URL is correct before clicking it.
- Do not click unfamiliar links sent to you by anyone including people you know.
- If you click it, check the adress thoroughly whether it's real or fake - pages like steampowered.com sometimes has "verified" along it's url. Check again - is the page real? Try writting it's adress in google, and see the results.

- Never trust a stranger if it involves you having to give items to him first. Always check their history first - tf2 hours, ammount of games owned, how old is the account, if he has valuable items in his inventory, and whether he has any negative comment on any community site ( steamrep.com , rep.tf, backpack.tf, tf2outpost etc.)
Beware that steam profile +rep comments CAN be easily faked! It's good if a person has some of them (that means he isn't a new account) but there are other things that are more important to check first.

- If a stranger ever asks you to trade your item to a friend so that he can check something, then beware of a scam. There is no reason at all for you to trade your item to a friend in order for a stranger to check anything. (The scamm works like this - his scammer friends changes his profile page+nick+prof picture to look exactly like your real friend - then they invite you to shared conversation - you can trade with people in conversation, even when they aren't in your friend list! Same as if you were on a server - you could trade with people you aren't friends with too). They will trick you, or your friend, when he thinks he trades you back your item.

- Do everything in one trade window if possible. There usually isn't any reason not to do everything in one trade.
- Avoid accepting cash or anything outside the trade window, unless you are 100% aware of what you are doing, the risks, and and how do possible scamms work (seek guides on pages like steamrep.com )

- Beware of impersonators (!) of trusted middlemen or of trusted traders. Many scammers will try to look like someone else, including changing their name, picture, info and id. All links from their profile might lead to the original person - because you can write anything on your profile. Always check via his ID if possible.
How to get someone's ID - go to their profile page and you can see it in their adress bar - in my case, it is http://steamproxy.net/id/antosheek/ - ID is the part after /id/ (in my case it's "antosheek".
Always check for spelling of those! An impersonator of mine could use id like "antosheeek" or "antcsheek". See the difference? Copy their ID in pages like backpack.tf or steamrep to see who that person really is.
(If you don't see the ID adress in steam client (you always do see it in browser) then go to your settings - interface - tick "display steam url adress when available")

+ look for more guides how to secure yourself, e.g.:
http://forums.steamrep.com/forums/steamrep-guides.22/