I get the idea but the method is far too extreme. Having NPCs refuse to buy player made items takes a lot of the incentive out of certain crafts.
Why? Because, in PS people do crafting for two reasons: to get better at it, and/or to make a profit. But the problem is, the number of items you need to craft to level up is FAR more than the number of items the player base is ready or able to use. Trust me on this. I have an alchemist character who has stacks and stacks of potions in storage which I have tried selling, but nobody's buying. I had another character who tried blacksmithing, but for the early levels there were no customers because nobody wants to buy low to medium quality weapons when other players are churning out 300q weapons all the time. There just aren't enough players around to want to buy all the crafted goods. Also for weapons and armor, most people only need one and they're set for a long time. So demand is very low because everyone who needs good gear already has some.
And if you can't sell your goods to NPCs, and players don't want them, you're stuck with a crapload of useless junk. What's the point of even practicing your trade at all then? You're not getting money from selling your product, which means you can't afford to pay other characters for raw materials. Alchemists and leather workers won't be able to pay hunters for animal parts and hides, so they'll have to either gather those themselves or give up their craft. Smiths will have to mine their own ores since they won't be able to afford to pay miners.
So, you can see how this actually leads to less interaction between characters, because it makes it the intermediate crafting trades (going from raw material to product) essentially receive no or little pay.
The one thing I agree with, though, is NPC prices may need adjusting to encourage player trading. For example, part of the reason nobody is buying player made potions is because NPCs sell greater health/mana potions so cheap. Why buy high quality potions for 1000 tria when you can buy 50q from NPCs for 200 tria and they work pretty much just as well? The prices should be adjusted so that it is indicative of how effective the potion actually is. Right now a 50q greater mana potion restores more than a high quality regular mana potion, even though the latter is 3-4 times more "valuable" according to NPC prices. If the price was proportional to the amount of mana restored, buying potions from players would be a much more worthwhile.