There has been a really sizeable, maybe even vast, increase in the number of shooters and gun enthusiasts in the last decade or two.
While many of them are drawn to the hot new current production stuff, many quickly catch on to the fact that there's a lot of cool variety in the gun world, far beyond the latest AR variant or polymer autopistol. And they quickly pick up the idea that a whole lot of really solid, very cool guns were developed in the previous century and aren't made any more.
More shooters wanting a great old Speed Six, combined with the fact that there haven't been any more made in decades, means higher prices for those that are for sale.
Basic economics: Fixed supply + increasing demand = price rises
If Bill and Ted both want a Speed Six, but there's only one Speed Six in the store, say with a $350 price tag. Bill offers to buy it, but before he can whip out his wallet, Ted says, "I'll give you $375 for it! If Bill says he'll beat that by $20, and Ted counters with another $30...well, eventually one of them decides it just isn't worth any more money, so the other one gets it. And that day, in that store, the price of a Speed Six was ...whatever the highest price one of the two most excellent pals was willing to pay.
If there's 10,000 Speed Sixes out there in dealer's "USED" counters today, and there's 20,000 shooters who'd really like to have one, then the average sale price for one is going to rise until the 10,000 guns are in the hands of the 10,000 new owners willing to part with the most cash to have one. And, given inflation, that's probably quite a bit higher price than they sold for new!