+Rep for +Rep

To fully understand rep, you have to go all the way back to 2009's Sniper vs Spy update for Team Fortress 2, which introduced two things: an item drop system and hats. This was way before the existence of the Steam Community Market, Steam inventory, or even trading.

Hats were pretty much an instant hit, and everybody wanted one. Since there was no trading, people took advantage of the item drop system's random nature and simply idled for their hats. Apart from regular hats, there were also special promo hats like Bill's Hat, Max's Severed Head, and the Earbuds, and they were only available for a limited time, plus you had to do crazy, reckless things like logging in on an OS X system, or pre-ordering brand new games.

When the Mannconomy update hit in 2010, the TF2 economy was basically already set up. There were thousands of common hats, plenty of promo items, exceedingly rare special items, keys with which to open the crates that you hoped would give you said exceedingly rare special items, and of course this incredibly strong, pent-up desire to bypass that stupid item drop system and acquire all the hats you wanted right this instant.

But prices on the Mann Co. store were steep, the purchased hats were "dirty", and availability of the rarest items restricted to crates or other players' inventories. Naturally, people turned to offering their money not to Valve, but other players, and where there are buyers, there are sellers. That's how Paypal trading started.

Since Paypal has no proper escrow system, great emphasis was placed on developing one's trading reputation. Paypal trading and reputation went hand in hand. Nowadays, you can't really do the former without enough of the latter, and you can't have the latter if you don't do the former. Back when everyone's rep was more or less at zero, however, people networked with fellow traders and developed an unwritten form of reputation before recording everything and setting them down in stone. It stands, therefore, that the oldest, most well-established sites have the most stringent rep rules, and anyone's rep there is the most trustable. Examples include the SourceOP forums, TF2-trader, backpack.tf, and bazaar.tf. To give an example of the opposite: profile comments are subject to barely any rules and any rep there should be simply disregarded. Steamtrades rep is also pretty bad, given that people just write "+rep good trader" and leave out all the vital information.

For more information see http://forums.steamrep.com/threads/what-constitutes-reputation.12391/

Info by: tnn21 (reddit)