It sounds great! They'll send you money, you do a test purchase with part of it (sending the surplus back to them) and you get to keep the lovely thing you purchased. It sounds too good to be true!
And, of course, it is.
The working of this scam depends on two things.
1) It takes a bank several days to process a cheque, and that includes a cashier's cheque.
2) If you send money via Western Union, it's immediate, and there's no way to undo the transaction.
So. You sign up, giving your name, address, and bank details (and there's lots of opportunities for additional scams using this information, which is going to be sold to other scammers), and you get a cheque, drawn on the National Bank of Ruritania, for £1800. Or some other bank, or some other amount. But it's a bank cheque, a cashier's cheque, and they're good as gold, aren't they?
Not always. This one is forged, because actually the "National Bank of Ruritania" doesn't exist. And you pay it into your bank, and you're credited with the £1800, and everything looks rosy, and you draw out the £1200 that you wire via Western Union, and you spend the other £600 on a lovely iSomething, and you're laughing.
Until the cheque bounces. And your bank isn't going to take the loss, no way. They're taking that £1800 out of your account, and if that puts you into unauthorised overdraft, there's a hefty fee for that, plus hefty interest rates, and don't even *think* of taking out a "payday loan" for this at 5000% APR.
And you go back to Western Union, and ask for your £1200 back, but WU explain that all transfers are irreversible. So you take your lovely iSomething back to the shop, and if you're lucky, you'll get your £600 back, so you're only out £1200.
There's lots of variations on this, of course. Anything that involves you receiving money from a complete stranger is probably a scam, and if it also involves you sending money back to them via Western Union (or similar service), it's definitely a scam.
So what should you do?
Easy - delete the email.