SE needs to copy Atlantica Online's Market.
It's the best out there, you can even search for craft recipes and it gives you the amount you will spend (by giving you the math for all items) at that time with current items posted.

Also, lots of items have 'npc' stock price so people normally post the price below the npc one to make sure it would sell.

And also, you buy the amount you want, independently of how many the seller added (up to the amount that specific seller has, of course).