Are the providers at g2a reputable?

Eg

https://www.g2a.com/call-of-duty-modern-warfare-standard-edition-xbox-one-xbox-live-key-europe-i10000188236010

The 45 euro are not offered as a "buy now" option… Is that a reason and are the providers on the site always serious or do you have to "check" them?

G2a is a marketplace similar to Ebay. You therefore have nothing about the provider. This can be serious, but also stolen key or paid with a stolen credit card or something. I would be careful. If the source is illegal, this can only be noticed in a few months and deactivated whether you can still get replacement.