How can I clean PSP games?

I bought a lot of 9 psp games on ebay recently and 3 of them do not work. Even though the first sentence of the auction says that the games are guaranteed, I might not be getting a refund. In case this happens, what can I try to get these games to work? The seller said that the games worked when they left the house, and I believe that. Ratchet Size Matters let me create a save file and went to a black screen. Spiderman FOF let me play the first level, which lasted about 5 minutes, and then did not work again. Mortal Kombat Unchained never worked at all. What should I do in case I do not get a refund?

I don't think there is a way to clean PSP UMD's, it'd most likely be very difficult if there is. But its also quite hard to dirty up a PSP UMD at the same time, I don't know what to tell you.

Are any of these good?

http://www.amazon.com/s/ref=nb_ss_gw?url=search-alias%3Daps&field-keywords=psp+cleaner

Well, try the link below, it might be worth it, if you have a lot of games.

PSP Disk Cleaner

WEll GAME do disk cleaning in-store

Im not sure if they do UMD's as well

its something like £3 per game

www.game.co.uk