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Pro One of the only ways to sell second hand game keys
If you have product keys for games that you don't need, you can sell them on Kinguin. It works similarly to Ebay, but it's a marketplace dedicated to keys.
Pro You build up a little credit per purchase
You get a few cents of credit per purchase you make, if you have a Kinguin account. That can make some difference after a bit.
Pro Usually doesn't get cheaper
Websites like Kinguin and G2A are usually the cheapest way to get game keys. There are tricks like using discount codes for -3%, -5% or even -7% discount. These keys can usually be easily found through a simple Google search or through a Twitch streamer who's partnered with Kinguin. Another trick is not to use the shield service. It's usually a waste of a dollar, given the fact that in reality issues with game keys are rare.
Con Sellers could possibly be shady
While keys are usually legitimate, there is often no way of telling how these keys were obtained. There are cases in the past where stolen credit card information was used to purchase a bunch of game keys and then those keys would be sold on platforms like this. So in that way it's similar to Ebay as well. However, it may be that it's blown somewhat out of proportion by gaming publishers who would earn more off gamers if they'd buy brand new keys.