IJBall: it really was not planned that some items would be unsellable to merchants. It just happens that if all the "rolls" are right, some items are so unique that they become "priceless". They are simply not sellable because merchants don't have that much cash assets. This is quite common in RPGs.
One technique I used, which I am surprised is not mentioned, is to buy some things from the merchant to boost his gold reserves, before selling that one big item. Then you can resell these smaller items back to him later or another merchant. Also, I have him repair my gear to boost his cash as well. Of course, this idea is broken if you raise your Mercantile skill so high that you are swindling merchants out of their gold!

I am not opposed to raising merchant gold levels in 1.04 a bit, if we can figure out a good compromise between what is helpful to the player yet realistic.