Which Series Have Gotten Harder To Collect Over The Years?

Seated Dollars. Premium quality bust material. Really probably most classic series when the prices were retreating. The premium stuff usually goes into hiding during soft times in the market

I would say I have to agree that Seated and Trade Dollars have become pretty unaffordable for an average Joe collector.
( Not Coconut Joe or Joe2007 ) lol
I have seen Twenty Cent Pieces go up dramatically, especially the 1875 CC and Standing Liberty quarters are hard to afford in AU and MS state where they were much easier to find and buy 20 -25 years ago.

All pre-1830 issues of every series, especially the copper.

I would note that the 1830-1870 period always have the same easy dates available all the time with anything slightly scarcer being held forever without returning back on the market.

Seated dollars have always been tough, and prices are finally getting to a more honest position.

You didn't ask, but the most under priced series, by far, are the classic silver commemoratives. They are at the very bottom of the price swing.

I agree with everything everyone else has said. Seated and Trade dollars as others pointed out seem to have increased in value by a lot.

The one I wanted to mention though are Carson City minted coins. Even the higher mintage coins are going at a premium now compared to others of the same type. A nice original circulated CC half is a great coin that immediately inspires thoughts of the 'wild west'.

I was in the market for those early S Seated quarters up until 10 years ago omegaraptor. Since then the prices have quadrupled. Oddly, this has made them more available than they used to be, but from the expensive specialist dealers. The less affordable they become, the easier they are to find.

Though it's not a classic series, I see California gold fractionals as being overlooked right now. They have the same regional interest and are as scarce as the S quarters.

Seated Liberty Dollars; uncleaned Bust & Seated coinage of any type with original surfaces; mid-grade to Uncirculated Barber half dollars; better grade Flying Eagle cents.
The prices on 19th c. gold from Charlotte, Dahlonega & Carson City seem to only go up and up...

I also agree that Trades have gotten harder to collect due to the massive influx of fakes, which has pushed the market ahead on certified genuine examples.

Easier to collect these days, in my opinion, than they used to be, would be the Two Cent and 3 cent (nickel) pieces, both of which seem to be suffering from a stagnant market and a surplus of coins available. There is also no shortage of common date Indian Head cents, V nickels, and Barber coins in circulated grades, and you can pretty much put together a partial set of those coins (minus the keys and semi-keys) in a single afternoon.

If you ever step outside the realm of US coinage, both Spanish (pillar/columnario type) and Mexican federal issue (cap and rays) 8 Reales are on fire, and have been, especially certified problem-free coins. A combination of lots of problem coins and lots of Chinese fakes is combining to make the "real" reales climb in price nearly every month.

I was collecting in the 80's and Commems, Walkers, and Morgan dollars in ridiculous high grades were considered "investment grade coins" and it was hard to find them at less than ridiculous prices. Stuff like Draped Bust quarters, halves, dollars in VF/XF did not bring big money as these were not investment grade and only collectors cared about them. Well, flash forward to 2017. The Morgan dollars and Commems collapsed, some trading at 1/5 of their 1989 prices. Draped bust dollars in XF were around $750 and are now $3,500+.

Conder1, if you inflation adjust your 1972 $10, it comes to $59, and the Dec 86 (I used '87) $1000 comes to $2,200. I can buy MS common Morgans for <$50, although the 93-S is way up there, as should be expected for a key date to surpass the rate of inflation. So from an average "Joe Collector" standpoint, I think it's easier now then it was for Morgans, until (like with every series) you hit the key dates.