Selling stuff on Ebay

Ebay has tutourials that will take you throught the process step by step. Be sure you understand it all before listing the first item. You can ask any specific questions you might have at Seller Central, an ebay discussion board. Experienced ebay sellers that post there are always happy to help. 

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I think the best selling HIGHEST PRICE ITEM is the unique one of a kind item that you cant get anywhere else. 
however, if you are referring to any item that has VOLUME SELLING that means whatever is in fashion at the moment. For this you would have to do a trend analysis and understand it will change all the time

Other than selecting the category and seeing whats on demand it would be good to see what items are being shipped from china. If they can mass produce it for that cheap there has to be a pretty big demand for them to get profit although i doubt many people can compete with them