Bought dozens of games and DLCs on Kinguin. Bought multiple gifts for friends also. Never ever had a problem, a delay longer than a few minutes or any kind of bulshitting on their part. I always remove their insurance fee and still, the one and only time I waited 10 minutes for the key to be delivered, they immediately answered my mail and solicited the seller.
Afaik, the whole "games bought with stolen credit" was an inflated bullshit which only really happened on a couple sites. The majority of those key? They come from mass sales of keys that developers directly agree with, some on the false assumption that they will be sold at the same price as they are on steam. Some of them, a bit more business savvy, understand how having a few thousand cheap keys sold by those sites actually helps you in the long run, exactly like piracy in the end helped some games that would otherwise never have been known.
In any case, this is just my 2 cents. The only thing I'd like to state over and over is that I've been buying from kinguin for the last 5 years, multiple times, without ever incurring in any kind of problem.