What the market deems it to be sellable at is the value of the item. If the item is selling well at 200k and it only takes 60k to make the item, 200k is not well over the value of the item if it sells at 200k consistently. If I undercut to 60,001 gil and hold it there, no one else is going to make any gil until I move out.
When individuals with a small supply of goods come face to face in competition, demand will eat them both out, or in your example you can buy them out but when someone who has a large supply comes in to competition with someone with a small supply of goods, you can't buy them out.
I'll throw up 3 items at the minimal profit margin and you'll buy them thinking your going to make profit off me because I've priced them so low but what you don't know is that I'm about to dump another 3 on to the market. You buy and next thing you know, I have 3 more at the same price on the MB. You can't make any profit until I move. You can attempt to find out how big my supply cache of goods are at your expense but most people are logical enough to realize they were just converted from a competitor in to a consumer.
We've seen it in action for as long as WoW has been around. Go play the auction house there and look around, you'll notice people using the listing fee as a tool to lock people out of the hotter/higher end markets.
Here, without listing fees, you can compete fairly all the way down to the minimal profit margin, against any size merchant, big or small.
