i don't think there should be any guarantee of how much profit you can make. By automatically making it so you can't price any lower than 10 or 20% higher than the npc prices, that's basically guaranteeing you that much of a profit instead of allowing the economy to work out itself.
ranchers have the issue of far too many players undercutting pet values, we have no guarantees of being able to sell pets - much less at a profit.
like merchants, our economy is based on supply and demand and to have it any other way is to create false inflation. =/
my suggestion to you personally would be if you can't sell something at a profit, don't restock it. figure out what items you can sell easily and for profit and stick to those. (ie items you know quests ask for, not items no one has a need to go searching for.) falsifying the economy as you are suggesting isn't the answer.