You can replace the locks that keep the JoyCons attached with metal ones. It's probably been dropped at some point, causing the locks to chip. Look on eBay for joycon buckle alloy locks. It's a pretty simple repair.
The back plate is simple to replace. Unscrew the center screw on the joycon rails (the other screws just hold the rails in place), and the rear screws. There may be 2 screws next to the USB-C that need unscrewing as well, don't remember. Check a YouTube disassembly video. TronicsFix may have one.
You'll need an Y00 triwing screwdriver, don't try to use any other size or risk stripping the screws. That size is tricky to find, iFixit sells Y00 bits for $2.99 though.
The screwdrivers you get free with replacement shells are NOT Y00 in my experience. They are Y0 and will easily strip the screws, I found this out the hard way. Not recommended.
If you want to replace the touch screen digitizer as well since it's scuffed, that requires a lot more effort. You need to almost fully disassemble the Switch to get at the ribbon connector for the digitizer, and you'll need to use a hairdryer on the front to soften up the adhesive to pry the digitizer off. Fully doable, just a bit finicky and time consuming.
How much did you end up paying for it?