How did you pay for it?
If it was on ebay or paypal you can file a dispute. Takes a few weeks but you get your cash back.
If it was sent by royal mail, find your local sorting office. Sometimes the 'You were not in' cards are lost (or in our postmans case, he doesn't bother with them! Git!) It might be worth checking it isn't sitting there.
If it was thru this forum for example, get in touch with the seller. Look back for their phone number etc.
If you sent a cheque, some accounts let you cancel it months after its been cashed, speak to your bank.
Best trying to sort it out off the forum if you can. Could be an honest mistake and coming on here slagging someone off puts them on the defensive.
I've had this happen to me on here, luckily I paid with paypal and got my money back. Its annoying the guy didn't have the guts to answer my Pm, email, text or phone call but you just kave to leave it in the hands of Karma! :laugh: