Unfortunatly you cannot do that.
The Devs decided that a good idea was to have severely restricting carry weights (mostly because some items are ridiculously heavy) and that merchants would only buy a certain amount of items from a certain type of items.
The result is that if you lower the value of the items then you'll just end up not having money, not because you don't get the items to sell, but because the merchants just do not buy in enough quantity... that and carrying them all becomes a huge problem as well.