only undammaged items are acceptable for trading; the npc only takes undammaged items and those without metadata for his inventory

README.md

@@ -87,3 +87,13 @@ yl_speak_up.replace_vars_in_text = function(text, dialog, pname)
 end
 
 The replacements will not be applied in edit mode.
+
+
+Trading (simple)
+================
+The NPC can trade item(stacks) with other players.
+Only undammaged items can be traded.
+Items that contain metadata (i.e. written books, petz, ..) cannot be traded.
+Dammaged items and items containing metadata cannot be given to the NPC.
+The trade can be added through the edit options menu when talking to the NPC
+as owner.

inventory.lua

@@ -88,6 +88,31 @@ yl_speak_up.load_npc_inventory = function(n_id)
 			if(not(yl_speak_up.may_edit_npc(player, n_id))) then
 				return 0
 			end
+			if(stack:get_wear() > 0) then
+				minetest.chat_send_player(player:get_player_name(),
+					"Your NPC accepts only undammaged items. "..
+					"Trading dammaged items would be unfair.")
+				return 0
+			end
+			-- items with metadata cannot be traded
+			local meta = stack:get_meta()
+			for k, v in pairs(meta:to_table()) do
+				if(k ~= "fields") then
+					minetest.chat_send_player(player:get_player_name(),
+								"Your NPC cannot sell items that contain "..
+								"additional (meta-) data.")
+					return 0
+				else
+					for k2, v2 in pairs(v) do
+						if(k2) then
+							minetest.chat_send_player(player:get_player_name(),
+								"Your NPC cannot sell items that contain "..
+								"additional (meta-) data.")
+							return 0
+						end
+					end
+				end
+			end
 			return stack:get_count()
 		end,
 	        -- Called when a player wants to put something into the inventory.

trade_simple.lua

@@ -43,6 +43,13 @@ yl_speak_up.can_trade_simple = function(player, count)
 		-- trade not possible
 		return 0
 	end
+
+	-- used items cannot be sold as there is no fair way to indicate how
+	-- much they are used
+	if(  trade_inv:get_stack("pay", 1):get_wear() > 0) then
+		return 0
+	end
+
 	-- all ok; all items that are to be sold can be taken
 	return ItemStack(trade.npc_gives):get_count()
 end
@@ -172,6 +179,10 @@ yl_speak_up.get_fs_trade_simple = function(player, player_gives, npc_gives)
 		if(not(npc_inv:contains_item("npc_main", trade.npc_gives))) then
 			trade_possible_msg = "Sorry. "..minetest.formspec_escape(trade.npc_name)..
 				" ran out of stock.\nPlease come back later."
+		-- only new/undammaged tools, weapons and armor are accepted
+		elseif(trade_inv:get_stack("pay", 1):get_wear() > 0) then
+			trade_possible_msg = "Sorry. "..minetest.formspec_escape(trade.npc_name)..
+				" accepts only undammaged items.]"
 		else
 			-- put a *copy* of the item(stack) that is to be sold in the sale slot
 			trade_inv:add_item("buy", trade.npc_gives)