I've thought about this a lot. I always try to live by the golden rule. I've sold some things locally during this last year that were basically duplicates, like Frankenstein ARs that I no longer use. Is it a fair thing to sell one of these for $600? No one wanted them for $450 a year earlier. Does it matter that I was out of work at the time? They were gone in hours. That is also less than it would cost for me to replace them, and roughly what I had in them plus or minus.
I usually only buy; not because I'm hoarding....but because I find something interesting or have a specific use for it. I sometimes buy for less technical friends that have a shopping list and I pass it along for exactly what I paid. I don't agree with gouging, so I don't sell specific items when things are crazy that would feed this. For example I have a laaarge supply of 22lr and small rifle primers; I could make a lot of money, make a lot of people happy, and replace it later. Would it be fair to double my money?
When you sell stuff that you've had 20 or 30 years, are you supposed to always sell for what you paid for it? I think I paid $89 for my once a lifetime CMP M1 garand, and $400 for a SC 1903A3. I have bought Mosin Nagants at $69 and K31s at $89 if I remember correctly. I think the tins of 8mm and 7.62x54 were $30 or 40. I prefer to check current prices (and I don't mean current, inflated panic prices) and set a fair price where both parties are happy.
That said, I have no problem with people asking what I consider to be crazy money like $.10 or $.15 a round on 22LR, or $100 or more a thousand for regular primers. I just laugh and move on. It is an option for someone with more money than time....or someone who just has to have them. We are talking about a sport after all; something that requires disposable income. To me it is almost something like complaining about membership fees at a country club or the cost of boarding a polo horse. I recall some comedian who added up the price per pound for venison from hunting.