I .. don't really understand this. Is the suggestion that users can only sell items in their shops for close to NPC shop prices?

Yes, the point of a user shop is to make sP but -

1. Items selling for less than the main shop price tend to come from quests and vending. I get a lot of these items from Pete and Cursed Quests but they give me sP along with those items. The Wizard also gives out sP as well as tokens which I vend with. The items are a bonus on top of the sP and I've made millions from selling them over the years I've been here.

2. The really cheap items are great for doing quests because you can make more sP from buying them rather than buying from the main shops. People are only prepared to pay a lot of sP for items if they are for treasure chests, galleries or special quests etc.

I'm so confused as to why you think this would improve the economy. It's far too controlling and would increase server strain due to the influx of items. The sp sink aspect is actually way more minimal than it seems and is kind of addressed by the recent changes to haggling and the addition of the 15% Discount buff. (The aforementioned change is - in my opinion- the very best thing that could have happened to the economy. Subtle, effective and motivating). The changes away from data-heavy programing have been so wonderful.

I'm also slightly offended that you discount the opinions of 'wealthy' users, since they are clearly the ones who understand best the way of making (and spending!) money on the site works. They are active and enthusiastic users, many with years of experience with the ups and downs of Subeta. Why on earth wouldn't they have valid points?