Depends on your point of view, I guess.
eBay relies on a system of 'caveat emptor' - buyer beware. If the goods described by the seller are not what is sent to you, or are otherwise faulty in some way then I would think it reasonable to leave negative feedback after first contacting the seller. However, if the goods arrive exactly as advertsied, promptly, etc then you can't go leaving negative feedback if they are as advertised (this includes if the seller puts 'I haven't tested it, it might not work...' kind of stuff).
If it doesn't work when the seller claims it will, then you should contact the seller first and give them the right to amend things. If they can't or won't then you have every excuse to leave negative feedback, IMO.
Feedback is more about the overall level of contentment with the sale, and not just whether you have bought something that ends up not being able to do the job you believed it might do. We have all seen scams on eBay, and may even have been caught by one, but at the end of the day you are not buying from a retail outlet and you are not covered by the same rules as you would be if you were buying from one. If you buy a tuning box that claims to add 90BHP minimum to your power levels, and it only adds 10 (or whatever) then you have grounds to complain. If it does what it said it would do, then you don't. Similarly if you buy an item advertised as 'new' and it is clearly not when you get it, then again, leave negative feedback.
I'd think the seller would be within their rights to respond to any negative feedback you left in a derogatory way if you simply ended up buying something which you found didn't do the job you wanted it to do - a bit like buying a screwdriver because you thought it would enable you to tighten a nut and finding out the hard way that it won't. That isn't down to the seller, that's down to you...
