Falas Selvayn at the Ramshackle Trading Post seems like he is supposed to sell the unique weapon Glass Bow of the Stag Prince. But since it is often the best weapon he has on him, he equips it instead. If you lure enemies towards him, he switches to melee. After that it is possible to buy the bow from him.
The player is almost certainly supposed to be able to buy the bow from him at all times, since the weapon is unique and Falas Selvayn has nothing else to sell that is all that special, even though he is such a unique merchant and has the special condition of only appearing at midnight. The warrior home Hendraheim and the mage home Myrwatch from the AE DLC pack also have special holders for this weapon.
Since the bow is purchasable and the player is supposed to display it and the bow's enchantment is only useful for the player, him not always selling it is probably a bug.