Grabbed a $12 digital scale from Wally World years ago for this exact reason and never looked back. Weighing works but the real trick is figuring out your target number per bearing size, I keep a little cheat sheet taped to the bench, 3g for the small trailer hubs, 5 to 6 for the bigger truck stuff. One thing nobody tells you, zero the scale with the cage on it first or you'll chase your tail trying to do math in your head. And yeah the comeback thing is huge, when a guy brings a hub back you either know you hit the number or you don't, no guessing. Only downside is you gotta keep the scale clean or grease gets in the buttons and it starts acting stupid.

I used to roll my eyes at guys who said just buy new, figured they were dealer shills or something. Then I bought a used $300 gauge off eBay that read fine on my test block and crapped out two weeks later on a real job. Now I get it, warranty and a real cert are worth the extra cash when you're betting on readings that keep people safe. That said, if you still want used, call around to the NDT shops near you and ask if they have trade-ins, since at least they'll check the transducer before selling. Lesson learned for me either way, for what it's worth.