Welcome to SaleHoo! :)
What to sell - 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. And the most popular at eBay doesn't mean they will be the easiest to sell. :( As a SaleHoo member, you have access to downlonable PDF guides that can help you decide what products to sell and which niche market to work on.
Get them from Link hidden: Login to view
I'll be searching for fitness products suppliers for you and I will update this thread once done.
All the best! :)