They aren't being bought for that much right now. There's a difference between people who are selling a product and a product being bought. Just because someone says they want a specific amount for a product doesn't mean that people are paying that amount for it. In general, mobile devices with tiktok are being bought for the exact same amounts that people spend on mobile devices without tiktok. It's about $50 to $200, depending on the age of the device.
It's a really bad idea to sell your old phone with tiktok still on it, though. In order for tiktok to continue being on the device and continue to work, you would be allowing a complete stranger to have access to personal information because it's impossible to remove all the information on a mobile device without doing a complete reset (which would remove tiktok in the process). Just some food for thought.