The only reasons that the internal parts could fail are either an impact had shocked them (as in a FRONT end collision) OR the car was ran without power steering fluid a looooong time and the parts got cooked.
$285 for a replacement isn't terrible if the unit works well. I have had great luck with junkyard units though. I check Diplomat and Gran Fury Cop cars first. I look for clean and dry housings that have NOT leaked. I have swapped several and only had ONE leak from the pitman shaft area. I have paid as little as $25 + core on half price day.