Please go to your “Inventory History” and see if that trade happened. If you see that Gameflip bot’s icon on the Steam website, go to its profile and make sure it’s a bot and not a real person (with friends, games, etc.) pretending to be a Gameflip bot.
If you traded with a real Gameflip bot, you could screenshot that trade and use it as evidence when asking help from support support@gameflip.com