I think there is a mod that allows you to repair items from the junk yard and use them. I've not tried the mod as I rarely bother with trying to repair anything. As far as I know you have to pay full price if you do decide to repair something.
EDIT: I couldn't find the mod, but someone suggested leaving the salvaged items outside the sim's house until something breaks inside. Then call the repairman and they will all get fixed. That might have been a glitch as the advice is 11 years old.
EDIT2: Here's the mod
https://modthesims.info/d/522749/update-1-sep-2014-salvaged-junkyard-objects-made-usable.html