Hi Sergio,
To tell you honestly, almost everyone is asking the same question but there is no specific correct answer to give, as the market is constantly evolving, changing. What to sell will greatly depend on your location (or location you want to cater to), the preferences of your customers in your market, the latest trends, the competition and of course the demand. A bit of market research might help you choose which products to sell. Also you have to remember that how you sell is just as important as what you sell. SaleHoo members are given access to downloanable PDF guides that helps them decide what products to sell and which niche market to work on. Link hidden: Login to view :)
Another thing you might want to check is your own shopping list, things you want to buy for yourself may just be the things other people are looking for as well.
Or look through print media - magazines, newspapers and even TV commercials if you see a product being repeatedly endorsed then you've found yourself a trend! Which could be a very viable market.
Have a read on these useful Education Resource on how to come up with creative ideas on what to sell at eBay Link hidden: Login to view
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Hope this helps!