One good practice is to put some form of generic identification on the pictures you post on the for-sale sites. This won't stop things entirely but it will make the scammers be hesitant to grab your pictures and use them on other sites.
Here's an example of what to do. A simple 3x5 card with your chosen generic I.D. placed in the picture would work as well.
https://www.gunsinternational.com/gu...101566295#lg-1