How do traders who resell determine the value of this game?

You can check AllKeyShop for rough value of games.

How should we know if they use a website and which one ?

Maybe question them directly....

It just means that there was a deal somewhere with a lower price, nothing really mysterious.

While I do not use it to buy keys per se, I do use G2A as a way to help value unbundled and bundled games. Best of luck in your pursuits!

Most simply use G2A or the AllKeyShop aggregator to tell the grey/black market price of a key. Then they either do the sensible thing and remove the extensive taxes and fees those sites put on each key and sell 10-50% cheaper, or they try to bank on people not knowing about the grey/black market or distrusting them more than an actual person, so they jack up the price up to double. A lot, and I mean a LOT of Rust keys were sold like that when it was first put in a high-tier Humble bundle.

Unless you are on barter.vg or lestrades, where some people have this very strange notion that original non-sale USD prices are the baseline, so they genuinely expect you to give them a brand-new 60 dollar game for two Dark Souls keys.