I just bought a used M2 512 GB Vision Pro. I thought I'd share my experience to help you and anybody else out who's looking to buy used.
At first, I was only going to go with swappa because there seems to be a more diligent process in making sure what's being sold isn't a scam. But after a while, I got more comfortable with eBay, where the better deals are tbh.
The biggest help in buying used now is the introduction of the AppleCare One plan. This means you don't have to look for a vision Pro that still has AppleCare Plus on it. If it's out of warranty, you can still buy it and add it on to your existing AppleCare One plan.
You can do all the negotiation beforehand before you place your offer. One additional step I did was with the person I bought from, after I finalized the purchase, I made them transfer the AppleCare Plus to me just to be on the safe side. This made the process easier for me to add it to my AppleCare One.
Lastly, the seller had mentioned they had a 21W light seal, but when it arrived it was a different size. So make sure you ask for a picture beforehand showing the measurements. Luckily, I was able to work out a partial refund, which is what I use towards getting the correct light seal size. And now a nice perk of the runaround is I do have an extra light seal for a $33w should anybody else ever want to try my device out.