The paperclip challenge is where you start with a low-value pet/item (can also be a group of them), and work your way up to a high valued goal through multiple trades. For example, I can start with a 2023 uncommon and try to trade my way up to a '09 very rare. Paperclip usually relies on the other user overpaying.
Here is the original paperclip challenge if you're interested:
Wikipedia page