I found this to happen also (leather armor +1 listed for 600g and charged 1500g) and I am pretty sure it is a design. IIRC in other BGs and divinity games (and elder scroll games now that I think about it) either your overall reputation (and/or alignment) or group rep would make the prices change. If you notice it does the same for items you are selling (i.e. lists one prices but sells for a much cheaper price). I am pretty confident in saying the price listed is the suggested price and what you actually buy/sell for is up to the vendor.
