Research in making money

Now selling your stuff you always want the most amount of money out of it , so two of the most popular  second sites are music magpie and eBay. I thought to try them both out and sold Gta 5 for xbox one , Music magpie offer me £15 for it but I put it up on ebay on audition and sold it for £29 that is nearly  double the about which Music magpie was offering me. What I am trying to say here is do your research before selling your item because you could make more money than  you think. ;)