It is "safe" but it may not be great for publishers/developers as I believe those sites take advantage of regional pricing in a way they are not supposed to, which is why they are not considered "official" key sites.
I use Cdkeys and Instant Gaming. Super safe and reliable over the last few years.
Recently tried Gameseal as well, they seem to not be a marketplace and deal directly.
I would avoid G2A, Kinguin, Eneba, etc, because they are a marketplace so you can get anyone on there. Sometimes it works other times you may have a problem on your hands. Any site that doesn't offer PayPal as payment should also be a red flag.
Use ggdeals, pretty good aggregator of pricing and has rating and other useful info.