Is Simply CD Keys trustyworthy?

Simply no.

According to WOT it is but i dont really know, it hasnt any user review so...

They're UK based and don't seem to sell cross region codes (RU + VPN Activated). However they do appear to be reselling bundle games (indiegala) and amd/nvidia codes. Not sure how they get their other AAA titles within that price range. They also seem to have limited quantities. So they're not ordering in bulk, but finding a few low ROW deals and then capitalizing on them.

So no, just on reselling bundle games they're not a legitimate store. Just an Ebay/generic-CD-key-site and unauthorized reseller.

Depends who you ask, we in the UK post about them all the time on Hotukdeals and so far as I know no one has had issues with them.

https://www.trustpilot.co.uk/review/simplycdkeys.com

a lot of 1 and 2 star reviews

Got 3 games off them without any problems.so I would say its fine to buy.
Got bf4,titanfall and new plants vs zombies. keys where in my inbox within 30 minutes