As per our cross-trading article, we forbid the sale or purchase of Adopt Me items for real currency. Alongside this, we forbid the use of third party selling sites, which you can find more on here.
If you spot an Adopt Me specific selling site, or an account selling Adopt Me pets on secondhand selling sites, you can report them to our team. It is the same process as reporting a player, except when the bot prompts you to, select “Other Reporting Options” which will give you the option to select reporting a third party seller.