Adds buy/sell safety check for Market Shops

* Adds a buy and sell safety check from the Market Shop type.
* Also includes an item validation check when loading items from SQL for Market Shops.
Thanks to @mazvi!

src/map/npc.cpp

@@ -4124,7 +4124,7 @@ static const char* npc_parse_shop(char* w1, char* w2, char* w3, char* w4, const
 			ShowWarning("npc_parse_shop: Item %s [%u] is being sold for FREE in file '%s', line '%d'.\n",
 				id->name.c_str(), nameid2, filepath, strline(buffer,start-buffer));
 		}
-		if( type == NPCTYPE_SHOP && value*0.75 < id->value_sell*1.24 ) { // Exploit possible: you can buy and sell back with profit
+		if( (type == NPCTYPE_SHOP || type == NPCTYPE_MARKETSHOP) && value*0.75 < id->value_sell*1.24 ) { // Exploit possible: you can buy and sell back with profit
 			ShowWarning("npc_parse_shop: Item %s [%u] discounted buying price (%d->%d) is less than overcharged selling price (%d->%d) at file '%s', line '%d'.\n",
 				id->name.c_str(), nameid2, value, (int)(value*0.75), id->value_sell, (int)(id->value_sell*1.24), filepath, strline(buffer,start-buffer));
 		}
@@ -4924,6 +4924,19 @@ static void npc_market_fromsql(void) {
 		Sql_GetData(mmysql_handle, 3, &data, NULL); list.qty = atoi(data);
 		Sql_GetData(mmysql_handle, 4, &data, NULL); list.flag = atoi(data);
 
+		std::shared_ptr<item_data> id = item_db.find(list.nameid);
+
+		if (id == nullptr) {
+			ShowWarning("npc_market_fromsql: Invalid sell item in table '%s' (id '%u').\n", market_table, list.nameid);
+			continue;
+		}
+
+		if (list.value * 0.75 < id->value_sell * 1.24) { // Exploit possible: you can buy and sell back with profit
+			ShowWarning("npc_market_fromsql: Item %s [%u] discounted buying price (%d->%d) is less than overcharged selling price (%d->%d) in table '%s'. Assigning to current sell value.\n",
+						id->name.c_str(), list.nameid, list.value, (int)(list.value * 0.75), id->value_sell, (int)(id->value_sell * 1.24), market_table);
+			list.value = id->value_sell;
+		}
+
 		RECREATE(market->list, struct npc_item_list, market->count+1);
 		market->list[market->count++] = list;
 		count++;