I struggle to find the added value in making a domain name more expensive that it would have been through a regular registrar. Do you provide additional services, such as curated listings of domain names you thought sounded good and were still available?
If I learn you're selling a domain just because it's not available for registration anymore, you are not adding value to my business, only increasing its costs.
Ahh. Yes, I just don't consider that "harmful". It is clearly opportunistic.
But, chances are that if I didn't buy it, someone else that planned on using it would have, and they wouldn't sell. The value add is that it's available to you at all, because I spotted the trend early.
The value you would add by making it available to me is value you would also add directly to the other parties intending to purchase and use it. So really, we're in the same boat as before as far as availability is concerned, except now I'm buying from you for a higher price.