Wow, so much bad advice in this thread. All from sig spammers, so that's to be expected. The legal consequences of spending coins which do not belong to you is that you're stealing them. If the owner of the coins can track you down (which is likely, since they can probably determine which exchange the coins were sent to, and the exchange will reveal your identity in response to a court order), you may be arrested and/or sued. You must contact the exchange and inform them of the error, so that they may attempt to identify the sender and return the coins. Depending on your jurisdiction you may be allowed to keep the coins if the owner cannot be found, but only if you have made a good faith effort to find them.
Is a site for buying and selling ONLY WITH THE SITE , is not a trading site in to particulars , the SITE buy and sell bitcoins and profit with the exchange diference, there is no phisica sender. (sorry i dont say the page, for security razons, while I think what to do. seems to me the BTC world is not so big and every owner of BTC sites has an acount in a forum like this)