What they are doing is cheque scamming, they send you a cheque for a greater amount than you are asking & when you deposit it it will look like it's cleared & they ask you to send the difference back but let you keep a bit extra for yourself for your trouble, then about a week or so later you get landed with the excess you've given them as the stolen cheque bounces from the overseas bank.
Do it then tell them you are having a nice holiday on their money, never give personal details like addy or home no.
Have a look at Scambaiters for tips on what to do if you want some fun.......I've had them make movies & take naked snaps just to humiliate them, here's one I had done a while ago.
http://www.youtube.com/watch?v=L0Urw...eature=channel

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Beaten up old 98 Gen 2 Liberty, 13.0, stock V3 STI running gear.
07 Transit motor home
69 VF & 70 VG Valiants needing full restoration
Yamaha Zooma 50cc rocket hopefully going with the Transit.