Similar problems in Australia with supply and demand — shop gets 100 boxes, announces it in email to members, and by the time most people get in the door it is gone! Frankly, I wish they'd keep their trap shut!
E100 is not a particularly attractive substitute for RVP50 in many special instances; the key is in the way RVP renders its greens and blues. I was not impressed how E100 renders rainforest scenery.
My recommendation with Velvia 50 / 100 (I use both) and RDPII et al is to
use it sparingly — for your best work, not snapshots or occasional use. When it becomes available, pounce.
Buy what you know you will immediately need, plus a little more for the freezer — do not hoard, as this deprives others from their opportunity to shoot.
I'm not worried how long my stock of RVP50, 100 and RDPII keeps in the deep freeze. Both 67s are loaded with RVP 50, probably yet to be wound to frame 1 from September loading.

There is evidence of people in Australia hoarding around 200-300 boxes of RVP50 (ProPacks) and on-selling to online suppliers. It is common to see a pack going for $300 on eBay — my advice for that is,
Buyer Beware.