Looking to build a website like ebay, need ideas ! plz help.

Hi, This is Mahesh here. I am from India and I am looking to build a website . I am a gamer myself and I found that trading games or selling used games here in my country is extremely difficult.
I am looking to build a website where it allows any individual to post a used game to sell or trade and whenever an individual successfully sells or trades an item , I would like to receive a minimum server fee. This is my idea.

How shall I accomplish this ?
I am thinking about something like ebay.
whenever someone wants to sell an item , he/she would post the item in the website . If anyone wants to buy the item he would purchase the item and the payment would go to me and after successful confirmation that the item is good and well received by the buyer , I would deduct my service charge and make the payment to the seller.

now the problem is , how shall the trade system work ? if someone wants to trade an item ,how can i be benefited from it ? How will I charge service fee for the trade system.
and I would require an online merchant account for online payment which cost alot. I don’t want to take such a huge step so soon and would like to start with cheap option like paypal gateway.

plz help me with me ?

any ideas or any suggestion on how this has to be build ??

Thank you.

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