diff --git a/inventory.lua b/inventory.lua
index 26c3e8d..2fd6c53 100644
--- a/inventory.lua
+++ b/inventory.lua
@@ -112,6 +112,39 @@ yl_speak_up.save_npc_inventory = function( n_id )
 end
 
 
+-- helper function for yl_speak_up.load_npc_inventory and
+-- minetest.register_on_joinplayer
+yl_speak_up.inventory_allow_item = function(player, stack, input_to)
+	if(not(player) or not(stack) or not(input_to)) then
+		return 0
+	end
+	local pname = player:get_player_name()
+	local n_id = yl_speak_up.speak_to[pname].n_id
+	if(not(n_id) or not(yl_speak_up.may_edit_npc(player, n_id))) then
+		return 0
+	end
+	local error_msg = nil
+	if(stack:get_wear() > 0) then
+		error_msg = "Your NPC accepts only undammaged items.\n"..
+			    "Trading dammaged items would be unfair."
+	-- items with metadata cannot be traded
+	elseif(yl_speak_up.check_stack_has_meta(player, stack)) then
+		error_msg = "Your NPC cannot sell items that contain\n"..
+			    "additional (meta-) data."
+	end
+	if(error_msg) then
+		yl_speak_up.show_fs(player, "msg", {
+			input_to = input_to,
+			formspec = "size[6,2]"..
+				"label[0.2,-0.2;"..tostring(error_msg).."]"..
+				"button[2,1.5;1,0.9;back_from_error_msg;"..
+					"OK]"})
+		return 0
+	end
+	return stack:get_count()
+end
+
+
 -- create and load the detached inventory in yl_speak_up.after_activate;
 -- direct access to this inventory is only possible for players with the right privs
 -- (this is an inventory for the *NPC*, which is stored to disk sometimes)
@@ -135,28 +168,8 @@ yl_speak_up.load_npc_inventory = function(n_id)
 	        -- Return value: number of items allowed to move.
 
 	        allow_put = function(inv, listname, index, stack, player)
-			if(not(yl_speak_up.may_edit_npc(player, n_id))) then
-				return 0
-			end
-			local error_msg = nil
-			if(stack:get_wear() > 0) then
-				error_msg = "Your NPC accepts only undammaged items.\n"..
-					    "Trading dammaged items would be unfair."
-			-- items with metadata cannot be traded
-			elseif(yl_speak_up.check_stack_has_meta(player, stack)) then
-				error_msg = "Your NPC cannot sell items that contain\n"..
-					    "additional (meta-) data."
-			end
-			if(error_msg) then
-				yl_speak_up.show_fs(player, "msg", {
-					input_to = "yl_speak_up:inventory",
-					formspec = "size[6,2]"..
-						"label[0.2,-0.2;"..tostring(error_msg).."]"..
-						"button[2,1.5;1,0.9;back_from_error_msg;"..
-							"OK]"})
-				return 0
-			end
-			return stack:get_count()
+			-- check if player can edit NPC, item is undammaged and contains no metadata
+			return yl_speak_up.inventory_allow_item(player, stack, "yl_speak_up:inventory")
 		end,
 	        -- Called when a player wants to put something into the inventory.
 	        -- Return value: number of items allowed to put.
diff --git a/trade_simple.lua b/trade_simple.lua
index 898a87b..2856cd2 100644
--- a/trade_simple.lua
+++ b/trade_simple.lua
@@ -722,19 +722,9 @@ minetest.register_on_joinplayer(function(player, last_login)
 			-- do not allow used items or items with metadata in the setup slots
 			-- (they can't really be traded later on anyway)
 			if(listname == "setup") then
-				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
-				if(yl_speak_up.check_stack_has_meta(player, stack)) then
-					minetest.chat_send_player(player:get_player_name(),
-						"Your NPC cannot sell items that contain "..
-						"additional (meta-) data.")
-					return 0
-				end
+				-- check if player can edit NPC, item is undammaged and contains no metadata
+				return yl_speak_up.inventory_allow_item(player, stack,
+					"yl_speak_up:add_trade_simple")
 			end
 			return stack:get_count()
 		end,