Hey, I'm just a guy on the internet who gets an absurd amount of friend requests from scammers daily. I've decided to make this guide to inform others of potential red flags and to give tips about avoiding scammers, I will add suggestions in comments if I feel they're good, and I will give credit to the person who informed me. With that out of the way let's get started.

Here are some warning signs that someone may be trying to scam you, not all of these are true 100% of the time, but they're usually true.
1. No Mutual Friends
This one is a rather obvious one, I usually decline friend requests from these kinds of people.
2. Poor English or Poor Grammar in general
This one is a bit iffy, some people may just genuinely be bad at whatever language they're speaking, however I've noticed a lot of scammers I have added usually have poor grammar.
3. Hidden Profile Details
Another obvious one. There's no reason to hide something on a profile unless you have something to hide. (Thanks Captain Obvious) People who hide profile details generally have things that they don't want to be revealed to their victims in an attempt to cover their tracks.
4. Having Every Achievement
This one is a giveaway mainly because people sometimes give themselves all achievements so they can seem like they've been playing a long time.
5. Name in a different language or name includes [website].[extension]
Different language can be a giveaway because it's in general odd, especially if combined with 2. Websites are generally just odd in general as well.
6. Anything that stands out or is odd in any way.
Simple enough. This can be applied to almost any list of red flags.

1. Generic "I accidentally reported you!" scam
They'll say some bull excuse, and transfer you to their alt. They'll then say on their alt you need to send your items to a "bot" of some sort, which is another alt.
2. Hey can you go to [website].[extension] and do [thing]. They will refer you to a website that will take your steam credentials by giving you a fake login prompt. They will hijack your account and do whatever they please after.
3. Changing items last minute or placing a "valuable" item in a flood of other items.

1. Don't add people with no mutual friends.
2. Always check profile details.
3. Check every item in a trade carefully.
4. Don't give people the benefit of the doubt.
5. +Rep means nothing,
6. Time, Achievements, Rep, and much more can be fabricated.

I hope this guide helps you in your future trades, I am open to suggestions and will add ones that I see fit. Have a great day and don't run with scissors!