A user by the name of BuRN Two (https://gameflip.com/profile/us-east-1:c75fd2a0-5916-4adb-96bf-227621f43a81) sold me a copy of Subnautica for $10. I told him my username and he added me (his steam name is gudi) and I got the gift, downloaded and installed it. It worked well so I gave him a positive rating. then about an hour later he blocked me on steam and game flip and the gift was revoked. here is a copy of the message I got from steam support.
The gift copy of Subnautica you received was revoked from your account because of a problem with the purchase. Steam cannot provide any details on purchases made on other accounts. To learn more you’ll need to contact the gift purchaser.
In order to play the game again you will need to either purchase a new copy or redeem a gift copy on your account.
Subnautica was redeemed on Jan 28, 2018 and revoked from your account on Jan 28, 2018
hopefully game flip can do something about this