It's not unusual for someone from out of the country to buy a UK registered car, however, you should exercise caution. Your bank account should have an iban number, this is to allow transactions from foreign sources. You only need to give them this number. I dont think they will pay cash, but if you can get it, then insist that they have it converted to sterling or you may lose out on the exchange rate.
Regarding the chassis number, most european countries are covered by the 5th directive, this means that they can obtain insurance cover for a vehicle not registered in their country of origin for the purpose of transporting it back home (it's a complicated law, so let's not go too deep), so him asking for that is not uncommon.
Just remember to exercise common sense, make sure you have full payment before you release the car, if in doubt, walk away from the deal, better to lose a sale than lose the car.