I rarely post here, mostly just a lurker. But since I'm directly involved in this situation, I'm going to try and add some perspective on the issue. I'm one of the people that got their keys banned and bought from G2A.com, now of course after this I learned my lesson and will never buy from a site like that again. So yada yada yada, let's get that out of the way.
Now here's the sequence of events that I've gone through so far, and as of right now I still haven't gotten a refund and I'm not planning to ask for another key from them.
I got my key banned the day after, and when you try to redeem the key again on Uplay you get an error message the key was banned for no particular reason. I then opened a ticket with G2A and they said you need to provide a bunch of screenshots. The list of the screenshots were your Uplay library, the error code, and an official response from Ubisoft. After reading that, I contacted G2A through their live chat system to make sure I had to provide the screenshots since I started doing some research on the issue. My case wasn't unique and they stated I had to provide the screenshots anyway. The CSR then told me the process after all the info is provided. Basically, after you provide the info stated above they contact the seller and make a judgement.
I opened a ticket with Ubisoft to get a response from them, knowing how much time a ticket response can take from any huge company I called their call center and got an expedited response from them to get the thing going.
I then submitted all the info G2A required and waited a couple days. Of course no response from them, so I went through live chat again. The CSR I talked to said he couldn't access the ticket so the only he could do was mark it as urgent for the department that handles these kind of issues. Now the purchase was made through Paypal so of course there's a bit of a cushion in getting my money back. But I've been pressing filing a dispute with Paypal to hopefully get the G2A customer service to expedite everything. As of right now, my ticket with G2A is marked as urgent and I'm giving them another 48 hours to respond. (48 hours from yesterday so 24 hours) If nothing happens I'm going to escalate it to Paypal and tell them the whole story.
The perspective I'm trying to give here is that as a consumer, I'm caught in a really messy customer service loop. Having to potentially deal with 4 companies to resolve this issue (I'm counting my bank in the off chance Paypal can't do anything.) Ubisoft should have realized this when they took action on this, as I'm never going to buy another Ubisoft game for a really long time and if I do, I'll wait until it's like $5. Of course there's minimal effort on their part so they can probably care less. Doing some research, G2A is apparently a reliable source to get cheaper games. (Take that in anyway that you will.) But their response in all of this is horrendous. Even after they released their official statement on Facebook, I still haven't gotten any response from them. What makes it that much more irritating is that they said they have to contact the seller, which if they did obtain the key from a stolen credit card, they're already long gone and probably won't respond to G2A at all. (I'm saying if on the credit card portion as some sellers may have used the region locked loophole. I realize it's highly unlikely, but I'm trying to cover all the potential scenarios.)
TLDR: From a consumer stand point. The consumer is stuck in an ugly customer service loop and it sucks.