Actually I would say you're safe, the worst that can happen is like how ubisoft pulled some keys. Those people got replacement keys and it's very rare in any case. You won't lose your money on G2A or Kinguin, they don;t want the negative PR - seems they get it anyway though...

This is from G2A "We have excellent business relations with our partners. We buy our keys directly from the distributors and this way skip many intermediaries. That is why our prices are so competitive, and we can offer games at much lower prices. If you buy at our web store, you can be sure you buy a valid and legal key."

And Kinguin,
How does KINGUIN buyer protection work? 1 Contact with Kinguin Support If your item isn't as described, there's an issue with activation, or item is blocked, just contact Kinguin Support. Give us a chance to make things right for you. 2 Give us some time. Our customer support specialists are here to help.

We can guarantee a fast
response.
We're available 24 hours every day.

3 We'll work with the seller to help resolve the issue. We'll contact the seller and ask them to resolve the issue. If we are unable to solve the problem, your case will get a special "covered by Kinguin Buyer Protection" status. Kinguin will refund your full purchase price directly into your account. Yes you are safe!

"We ensure your safety on multiple levels. First of all any seller who wants to sell on Kinguin must go through a very tough verification process. Our high-end technology and professionally trained staff enables us to filter trouble candidates in advance. Most of the current sellers are companies we have worked with in the past so we know them very well.

The second insurance layer comes with the fact that we release your money to the seller after 30 days has passed from your purchase. This gives you the power to execute your rights in case of any troubles or disappointment you might feel with your seller. Last but not the least your opinion matters too. Users evaluate sellers with each transaction by a positive, neutral or negative comment."

I've used kinguin a bunch of times and no issues here