It might be possible for something to cease to be 'new' but to then regain that status. Here's an example:
Let's suppose that someone buys something from Argos. On getting it home, and opening the packaging, they decide that it's not what they really want. 4 weeks later they sell the item through an ad in their local paper. I would suggest that the law would not regard the item as 'new', since it was then sold without the same consumer rights which were present when Argos originally sold the item. (e.g. any warranty would have 4 weeks fewer to run on it, than a genuinely 'new' item would have).
However, let's now suppose that the person who bought the item from Argos waited for 4 weeks before returning it under their '30 day refund' policy. If Argos then resold the item, it would (because it would have a newly dated retail sales receipt) be sold both in a 'new' condition and with the warranty that a genuinely 'new' item should have.
Hence, the original purchaser couldn't reasonably sell it as 'new', but Argos could do so.
Chris