I have recently replied to ad on gumtree.co.uk about an item and paid half the costs as a deposit the seller kept putting the dates back as to when I could collect the item and said I could wait or they would return the deposit. I replied after the weekend saying I had decided not to take it and the seller is now saying they will not return the deposit unless the item sells to another buyer, I don't believe this is fair, but I don't really no if I have any rights as it was a private deal?

Fair, maybe not, but you agreed to their terms of sale.
Problem here is the "deposit" and reasoning for backing out,
Now how did you pay this as there may well be a way out, although I would like to know more about this issue.

How long have they made you wait, how long has elapsed and why have you decided not to continue with the transaction.
Also what rough value is involved..