Best site / thread to identify the game's value on trading market

You can't buy games here, only give them away and receive as gifts (as the website name suggests).

I'd just google "buy <game name> key" and a bunch of grey market sites pop up whose prices are a good reference point.

SteamDB, IsThereAnyDeal, GGdeals, etc.

if you win a game here it needs to be activated on your account. not doing so will result in a ban from the site.
nudge nudge

[...] I want to start buying games on stamgifts [...]

Oh no, we don't sell games here. Game is bestowed to devout supporter by the great RNG god. Also, please fix your not activated win: https://www.sgtools.info/nonactivated/Patrichp

-> Steam profile of Patrichp : " ! : This Steam user has been linked to dangerous activity "

Ok, instant blocked ...