For the quality build, excellent sound, versatility, and uniqueness yes they are totally worth $299.
As far as what I bought mine for, all of the ones I have bought have had damaged or missing cords but were otherwise in great shape.
2 of them I bought 6 months ago had chopped off cords, but were in “A” condition ignoring the cable. The shells, drivers, pads, headband, sound etc were are mint.
My most recent pair (bought 2 weeks ago) had a chopped off cord AND missing pads. It also had a few nicks (ie overall “B” condition ignoring the pads and cable). The seller accepted my offer of $65 shipped.
Since the HD630VB are built like army tanks, I have had nothing but good luck buying the used ones. But then again, I have the skills and determination to do any repairs necessary to get them operational.
Buying new from a dealer or Amazon gets you a brand new product with full warranty (none of which I have). If anything is going to go bad at some point, it will be the cable. It is replaceable, but you have to use the proprietary Sennheiser cable. The other option is to do a 3.5mm removable jack (which is what I did on mine).
If I did not have the skills, tools, etc to deal with the used/damaged ones, I would jump on the $299 Amazon pair.
If you don’t like them you can send them back, or easily resell them for what you have in them.
These cans are truly underrated, and completely worthy of the Sennheiser HD family designation.
I wish
@Hawaiibadboy would do his YouTube review of them (he mentioned he would, but that was a long time ago). In suspect once he does, they will blow up and the prices will go through the roof via supply and demand. He has a loyal following, and stuff he recommends has a habit of blowing up. That’s just a prediction though.