So what's new?
ebay being the equivalent of a bazaar, you just need to remain mindful of what kinds of items are likely to be counterfeit, and whether it being counterfeit could be a problem per your use of the product. Flash cards and USB drives in particular aren't something you'd want to buy on ebay, an ebay seller cannot much if at all undercut a major supplier for such things, particular since memory density keeps rising so old stock costs more, not less per GB.
You also have to always take a description of "new" with a grain of salt. Could mean looks new, could mean didn't keep using it after it was tried out, could mean manufacturer reject, later deemed defective, formerly new product returned by customer for refund, unused but sat on a shelf aging when it has a finite lifespan (batteries, capacitors, printer ink cartridges, etc), or the most pathetic excuse of all ... "I didn't use it and claim I don't know of anyone using it and it doesn't look old so it must be new." "Oh, nevermind that the box was opened and some parts are missing, I never claimed you'd get anything other than what was pictured regardless of what always come with the new full product."
Keeping these things in mind, there are still a few good values to be had on ebay. Leave negative feedback any time you encounter a situation you wish you knew about before making the purchase, and be sure to read the feedback other customers have left.