У вашего броузера проблема в совместимости с HTML5
In this tutorial I demonstrate the use of the MarketplaceService and cover a few inbuilt functions that allow you to check if a player owns an asset or not then corresponding to the returned value, we give them a prompt to buy the item or print "player has asset" if they already own it.
Although I do not show this being used in a store like fashion; if you've watched any of my function centred tutorials then it shouldn't be too hard to prompt the player to buy the asset with GUI/ClickDetectors ...etc. As all you really need to do is copy and paste all the code in and re-tag 'Player' accordingly.