The merchants have a limited amount of gold (which is shown in their inventory list), so will not necessarily be able to completely match the value of your goods. Either trade for charms or other low weight expensive stuff you can use to barter with later (at a bit of a loss) or sell what you can, then exit and re-enter the BF to reset the merchants' inventories (assuming you have the
patch installed), including their gold reserves.
In any case, I never noticed a difference between what BF merchants offer for the same stuff. AFAIK the potion dealer should give you as much for armour as the armour dealer.
Unique or quest items, as well as equipment worn by the non-lead characters, will not show up in the inventory list to be sold. There are also miscellaneous items, like non-plant alchemy supplies, which can not be sold.