Since the axle is just a hard part, single piece of steel relatively unstressed, a visual inspection is enough to justify using a salvage part.I was going to say something about how common the axles are at bone yards but many folks don't like replacing bad used parts with more used parts.
Oh, the axle I replaced the bearing on was still running quiet and not leaking 40,000 miles later when some kid front-ended my '87 Country Squire. I've saved that vehicle for parts. Its 3.55 rear end is limited slip and I may transplant the center section section into my rust-free '89 Crown Vic one of these days-its already got the wagon's rebuilt transmission.
That and of course, I used a brand new bearing and seal.
For something like a water pump, caliper ,etc no way. New is the only way to go.