eBay is the garage sale of the Internet.
So yes - in one respect, it does reflect some trends.
However, like any garage sale you are following trends of used easy to ship goods. So realize it only gives you a read on a portion of the market. You would not buy a candy bar, a couch, a roll of paper towels, a bowl of fruit, etc - whereas a used Sega system or a collector's beanie baby would be perfect. Used electronics and collectables are the biggest sellers on eBay.