Thanks for the feedback everyone, I should just be patient. It's tough, I'm just starting out and there are so many toys!
Here's a somewhat contrarian thought. The only new, galvanized HP100s that are available are Faber, which retail for $335-$349, depending on the online retailer. (At least one actually lists them for $450 each, but that's an outlier.)
PST and Worthington cylinders are no longer manufactured and are therefore available only when someone decides to sell them, which makes for a fixed (and very limited) supply.
Yes, having 3/4" valves would be preferable. But, I have a couple of PST 120s with 7/8" necks, have dove them for years and the valves are fine. Any DIN regulator should fit.
$175 / cylinder would not be a bad price, even considering the presence of the 7/8" neck. If you are interested in them, making a lower offer does no harm. The worst thing that can happen is the seller says 'No'. Even if you don't get them, you gain some experience with buying, or at least negotiating for, used gear.
Don't be dissuaded by the spectacular deals someone else may have gotten. The issue is whether you want them, now, at a fair price. You could wait, forever, for the 'best price'.