After doing some extensive digging in our app store I was unable to find an app that allows trade-in functionality in your Shopify store.
However, it appears that an old app called Trade Orders once existed but is no longer available through the app developers. I was able to locate the app developers here and they currently offer app development services. It's worth a shot to contact them to see if they can code this functionality into your site. Otherwise, you can always visit our Shopify Experts page which allows you to hire a freelancer or agency for various tasks, including app development.
Let me know if this helps! If there is anything else I can help you with, please let me know.
Hey Thomas. I'm at a hackathon this weekend and we're looking to build an app by the end of the day tomorrow. Would you be up for a quick phone call to describe this problem? You can grab 15 mins on my calendar here: https://calendly.com/chrisjustin/shopify