Hello Alex - You post does not specifically call out which category of products you are looking to sell. So my advisory would be a little generic.
You mentioned you were planning to sell on Ebay. If this is the case, why not use ebay for your research along side the salehoo labs. Both come free for members. In ebay, got type a product you are looking for in the search function. Once you see a whole bunch of "best match" results showing, there might be a few key things you may wish to note -
a) All listings, Auction, Buy it now (These are right above the actual listings) - These would show you the live listing that best match your search description under each parameter above
b) Step 2 - Look at the different panels and options available on the left side menu bar on your page. Depending on where you intend to sell, you could possibly ebay.com only, listed on ebay.com, US only and so on. I sell in the US. So I would click US only.
c) There would be other product and shipping options to pick from. Select as appropriate.
d) The most of all - You would see a radio button / check box for Sold Listings. Click on this. Assuming you selected US in step B above and selected Sold Listings in this step, the result should show you all listings sold for the particular category you were searching. This should give you a fair idea of what to sell and at what rate under the specified category and should help in your basic research.
e) Go to ebay tutorials and review the different guides available. There would be a few guides that might advise you specifically on what sells in which season and might help you to align your sourcing decisions accordingly.
Not sure if this helps. Sure hope it does, given the time I invested in typing this response :)
Good luck and God Bless. Do feel free to ask any questions you have. My knowledge is free to share.