NEVER download anything sent to you by someone who you are not familiar with. A great many scams involve having someone download a program. THIS INCLUDES chrome extensions. DO NOT DOWNLOAD chrome extensions sent to you by people you just met.
Never trade with anyone who has a private backpack or profile.
Be aware that very new and very old steam profiles may be scammers. Scammers make new accounts frequently and buy old accounts to look legitimate.
Always check the number of hours a person has in the game they're trading in. A lot of scammers will not have the game installed, or have VERY few hours. If they DO have hours, this does not automatically mean they are legitimate, scammers are BUYING accounts with hours in a game specifically to fool people. Always do a full check on an account before trading with them.
A high steam level DOES NOT make someone reputable. Nor does having a valuable backpack.
DO NOT TRADE for wallet codes, CD keys, or games from unreputable people. These can be bought with stolen credit cards (which will get you trade banned) and are a COMMON SCAM.
As of recently (July 2016) gambling and betting sites have been issued orders from valve to shut down. DO NOT use any gambling or betting websites that you are linked to, even if they ARE legitimate, there is a possibility that your items will end up in a trade banned bot because valve took action.
In paypal trading, NEVER send your item until the money is IN YOUR WALLET. If they send you a picture or a link, it's a SCAM. The money is only sent when you actually have it in your wallet. Scammers will use fake invoices to trick people into thinking the money is on its way. There is no such system on paypal.
STEAM WALLET funds can NOT be transferred without the PURCHASE OF AN ITEM on the market. DO NOT accept an offer claiming you will receive wallet funds.
If someone adds you saying they want an item, and another person adds you selling that item, IT IS A SCAM. This is called the glimmer drop method. The scammer pretends they want it, but when you buy it off the market (from the scammer) they make profit on you and leave. Rather than accepting the current sale price as an item's value, look at the GRAPH to see what it has sold for in the past. Generally, AVOID 'rare' Dota 2 items.
NEVER, for ANY REASON AT ALL trade your item for nothing. It doesn't matter what they say, they are scamming if they want you to trade them your item. Whether they say it's for "item verification" or to "check if it's duped," you should ALWAYS be receiving an item you want in a trade. This includes trading items to friends. Don't send your item to a friend, THEY COULD BE SCAMMED TOO!
ALWAYS check the trade before accepting. Sometimes people will put in a bunch of stuff and remove one of the valuable items. ALWAYS MAKE SURE that every item you want is there.
Similar to the previous scam, ALWAYS check item quality. If you're trading for an item of a specific quality, for example factory new, or burning flames, ALWAYS make sure it really is what you think it is.
NEVER trust a middleman, admin, or anyone else claiming to be reputable until you've checked their SteamREP page using the method detailed in this guide:http://steamrep.com/tips/pretrade
For simple item trades (items only on steam, no cash or codes) DO NOT involve other people. An item trade is between TWO people, anyone trying to get your friend involved or get THEIR friend involved is trying to scam you.
One of the most pervasive and dangerous scams at the moment is the item verification scam. There is no such thing as a glitched or broken item, never give your item to people claiming to be admins for any purpose other than for MIDDLEMANNING. And if you use a middleman, ALWAYS verify that they are a real middleman via the steamrep pretrade tips.
No Steam admin will EVER add you, period. DO NOT TRUST anyone claiming to have power over your account. You own your account, and you own your items. It is your responsibility to keep your items safe. Extended reading: f-o-g.trade/guides/11