How to get good deals.
Here are a few handy links and guides to help you learn about turning a big profit when trading.
Some of these guides are in forum threads. Don't post in those as you'll likely get "yelled" at for bumping an old topic.

Quick history of sharking

One day on SourceOP they realized that you could rip people off by checking their playtime, scanners were not developed back then so "server scanning" was a tedious process of copy pasting.
The first server scanner never actually became popular, it wasn't that great but it was the first. "Up To No Good" quickly followed it and became wildly popular, so popular in fact that it's still on google's first pages despite being long broken...
Eventually someone on sourceOP finally completed their "collection" of 50 Max's heads then declared that sharking was bad and got SteamRep to start marking sharks as scammers and/or slapping warning on them.
Eventually people realized that you can't really tell a legitimate good deal trade from sharking because everyone's trying to get a good deal anyways so they un-marked every user marked for sharking and admitted marking them was a bad idea.

Quickstart guide

There are loads of guides out there on how to do profit trading and I've linked a few I gleaned useful information from but first I'll put up some core tips:
Don't be scared of people reporting you: Profit trading/sharking/jewing won't get you marked on SteamRep. It will however prevent you from becoming an admin on certain communities and will get you banned from some small communities/servers if found out.
Always stay polite when you are trading or in game, it is important to keep things cordial and civilized else your mark will likely run away.
Wait - I've had some trades where I had no intention of sharking at all but took a long time to find an item in my backpack, that's where I learned that people will put more items into the trade because they want to convince you to put yours up.
Images speak volumes - Users with no avatar, users unironically wearing a -meme of the week- avatar and users with default avatars are a lot easier to shark than users with original or uncommon avatars.
Groups - Certain groups on a user's steam profile are a sign that the trade will not go well for you.

So anyways a few guides written by others in no particular order: