Unable to overstock merchants

In Morrowind, one could "overstock" merchants. OpenMW does not allow this.

For example, a merchant could have five Comberries for sale and restock those when you close the barter window. You could easily increase their stock by purchasing the five berries, exit out of the barter window and immediately sell those berries back to them. The merchant would then have ten berries (five from you and five from the restock). Exiting out and buying those ten berries again would make the merchant restock to ten berries.

This way, you could "feed" the merchant items they would continue to restock in greater and greater quantities.

(RM-2473 from redmine: created on 2015-03-30 by Frank van der Waal, , duplicates #3482 (closed))