Since the amount of sites offering steam games has increased radically the past few years (as well as the amount of dodgy ones too), I though it wouldnt run to give you all some basic tips to make sure that you are not wasting your time and money.
First of all
How do we define legitimate resellers?
Any site which do have an official agreement with Publishers/Developers is considered to be it, including most of these major bundle sites too. To give you a quick rundown of some of these legitimate sites: BundleStars, Humble Bundle, GameFly, IndieGala GreenmanGaming, Groupees, Gameserve, Amazon, Nuuvem. fr.gamesplanet.com uk.gamesplanet.com de.gamesplanet.com it.gamesplanet.com es.gamesplanet.com www.gametap-shop.com micromania.gamesplanet.com fnac.gamesplanet.com dlgamer.com jeuxvideo.gamesplanet.com achat-jeux-pc.sfr.fr gameshadow.gamesplanet.com gamerunlimited.gamesplanet.com/ gamestop.gamesplanet.com www.mcgame.com www.centralkeys.com.br www.funstockdigital.co.uk www.gamesrocket.co.uk www.gamesrocket.com https://www.fullgames.com.br http://www.superduperbundle.com https://coinplay.io/.
Unauthorised resellers: now these are those sites you should be definitively vary of, most of the time they are offering keys (or steam gifts) from unknown sources .
Most important things to look out for when checking these sites
Price: as a general rule of thumb, newly released AAA titles are discounted around 20-25 percent, games that are a bit older /have been bundled before can reach a bigger discount. Using isthereanydeal.com is usually a good way to see if the discount offered by xy site is much different from the big named sites ones aka probably too good to be true (then again the opposite can be true too). There are obviously some exceptions but they are more often than not very rare.
for example: site offering you Five Night At Freddys with 50 percent discount : more likely legit since the game was discounted/included in bundles several times.
site offering you Dying Light for 10 dollars: suspicious
Site layout, general design: Legitimate businesses tend to put more effort into making a good looking website while dodgy ones usually prefer to throw a bunch of basic default templates together and calling it a thing.
Language: Any site using broken english/ full of spelling mistakes is not something that you would see for example in a general store and so you shouldnt expect any different from sites offering digital goods either.
WOT/any other sites rating: Thats a very important (!!) part, as they can be a very meaningful sign as to whether or not the site is legitimate. Do take these with a grain of salt however, since many shady ones tend to have many fake positive reviews too.
Shipping method: legitimate businesses tend to deliver the bought products through mail or by attaching it to your store account (if there is any). No legitimate site would EVER ask you to send them a copy of your personal documents or asking you to let them activate the game game trough Teamviewer/remote control.
That would be it for now, hopefully this will come useful to at least give you a general ideas as to what you should look out for when doing your shopping for steam games. I might do other tutorials too if there is need for it
Please let me know if you think this tutorial was useful and if I should add additional things that I may have missed.