I like this idea. Begging could be seen as a negotiating skill that reduces the cost of items from npc vendors and increases the cost of items you sell them. However, I don't know how this would function (Target NPC, if successful at begging, NPC reduces cost of items being sold).
Wouldn't everyone just make a beggar on their alt, and trade nothing on their main? Seems rather silly to me. Also, most shops would throw a rag tag beggar out of the shop, the moment they came in because clearly they don't have the gold to purchase anything and are more likely to steal an apple than buy one..