On the surface, it doesn't look like a bad idea. But I wouldn't want a random person and seller on Ebay to know that I am looking to buy his broken hardware wallet. Sure, he can't know that I need it for spare parts, but still. There probably aren't that many people who buy hardware wallets (new or old and broken) who are not involved with crypto in one way or the other. Maybe there are some who buy them, fix them, and sell them as used. But you are always taught not to purchase second-hand HW, so I don't believe there is a big market for them. Unless you ship it to a PO box, I don't want to order cryptocurrency wallets to my home address. Dealing with big companies and wholesale services is a bit different.

I am not aware of any vulnerabilities and backdoors that can be inserted and connected to the display. The chips and board are a different matter though, and you shouldn't use those parts unless purchased from official shops.