Apple laptop

i bought an apple laptop on line and i cannot access it because it needs a password

Did you try to contact Apple, or the store where you bought it? Is there anything in the manual?

its a second hand machine bought through gumtree, it came on its own with the case and no discs. ive phoned the guy and he says he is not responsible he just sells them (excuse me ) actually he took it in as part exchange. ive asked for a refund, no joy.

If you have your OS X Setup disc, insert it into the optical drive. On startup, hold down the "C" key. Doing this will force the computer to start from the setup disc. Once it boots into the setup, go to the "File" menu and select the "Reset System Password" option. This may be a bit different depending on which version of the OS you are using, but the option should be available under one of the menus. Once you reset the password on the hard drive, you should be able to log in to the machine.

Good luck.

EDIT: Sorry, saw your later past...you can use any OS X startup disc to do this, not just the one that came with your machine, as long as it is the same version or later than what is installed on the computer. I would be a bit concerned if the seller didn't include any of the media, and was not willing to supply the password. It could very well be that the seller was passing stolen goods, esp. as laptops are nearly the most widely stolen commodity you can find online. If you do unlock the hard drive and discover that the laptop had previous owners...it is up to you, but remember that receiving stolen property can make you an accessory to the crime in some jurisdictions, depending on the laws in that jurisdiction.