Between my hearing aids (Lumity L90), two Roger Ons, and various accessories (chargers, extra receivers, etc.), I’ve shelled out about $2500 for what would have cost more like $10,000 if I’d gone through a local audiologist rather than buying on ebay. However, it’s a crapshoot, and you may need to either have professional help or be pretty good at trouble-shooting yourself.
The aids were fine. They were sold as ‘barely used’ by an audiologist looking to offload them after a patient returned them post-trial. No drama there. I won them for a bit less than $2,000. I could have gotten the same thing for cheaper if I was willing to wait longer and look for listings posted by people who had no idea what they were selling (e.g. “hearing aids - power on, but can’t test” vs. “Phonak Audeo Lumity L90s - barely used, like new, guaranteed in working order”). I looked into ebay’s return policy before buying them, and if they hadn’t worked, I’d have had some protection and should have been able to return them for a refund.
The serial numbers are consecutive, and Target says the warranty runs from about a month before I bought them, which lines up with what the seller said. But Phonak says self-programming voids the warranty, so keep that in mind if you’re planning to do it. That didn’t bother me, because I knew I could buy 1-2 additional pairs for what it would have cost to get one with a proper warranty.
The Roger Ons were another matter. I bought a Roger On In for less than $100 from someone selling a whole bunch of them. The were listed as ‘for parts,’ but it said they powered on and the seller just didn’t know how to test them. The listing said nothing about licenses, but they appeared new (the photo still had the plastic cover on the screen), so I thought the odds were good that it would have two licenses. I understood I was rolling the dice, and decided it was worth it at that price.
It worked fine at first. It had two licenses, upgraded via the Roger Upgrader, etc. I was thrilled to have gotten a functional Roger On for that price. But a few days later, it started doing this weird thing where it wouldn’t turn off. When I flipped the switch to ‘off,’ it would power off for a few seconds, then restart. It would keep doing that until the battery died. It functioned perfectly while on, but that just wasn’t a workable arrangement.
I shrugged, figured I’d gotten my money’s worth just for the licenses, and bought another one from a different seller (also for less than $100). Same saga: it worked initially, then started doing the same won’t-turn-off thing. I eventually figured out how to fix them both, but that required a fair amount of experimenting and assistance from a professional mechanic (to whom I conveniently happen to be married 
). If I hadn’t been able to do that, I guess I’d have tried going through Phonak for the warranty, but everything I’ve read suggests that would have been a frustrating waste of time. Or I’d have taken it to a cell phone repair place at the mall to see if they could figure out it, but I wasn’t optimistic about that, either.
Your audi may be right that used aids often cost more than new ones, depending on how you plan to program them. When you buy new aids through an audi, the price often includes programming, follow-up visits, cleaning, warranty servicing, replacement doodads, etc. If you just bring in a set of aids you bought an ebay and still want the audi to do all of those things, then they’re going to charge you separately for all of them, and the total may well end up being more. But if you can handle most of it yourself and you want to go that route, it can be a lot cheaper. It’s also higher-risk, though, so you probably shouldn’t spend your last $1500 buy used aids on ebay. In that situation, you’ll be SOL if anything goes sideways.
Sorry for the massive wall of text! TL;DR: This can be fine and a huge cost savings, but it’s a crapshoot. Budget accordingly.
