Cheap way to buy Avios points (£500 per 100,000)

Just noticed that Avios are running a promotion in February 2012.
Exchange your Clubcard vouchers for Avios during February 2012
For every £2.50 in Clubcard vouchers, get 400 bonus Avios
That's on top of the standard 600 Avios you usually collect
And you can do it as many times as you like.
So for £250 of vouchers you can get 100,000 points.
Combine that with the fact there are a gazillion Tesco clubcard points being sold on eBay at any one time at roughly £2 cash per £1 of voucher and for £500 you could have enough points for a club world return to the USA!
I keep double checking my maths to make sure I've not made a mistake but it seems that other than the small risk of a dodgy ebay seller (which you can mitigate but buying small amounts at a time) it seems too good to be true.
What have I missed?
I wonder if there are any similar schemes for Virgin?
Exchange your Clubcard vouchers for Avios during February 2012
For every £2.50 in Clubcard vouchers, get 400 bonus Avios
That's on top of the standard 600 Avios you usually collect
And you can do it as many times as you like.
So for £250 of vouchers you can get 100,000 points.
Combine that with the fact there are a gazillion Tesco clubcard points being sold on eBay at any one time at roughly £2 cash per £1 of voucher and for £500 you could have enough points for a club world return to the USA!
I keep double checking my maths to make sure I've not made a mistake but it seems that other than the small risk of a dodgy ebay seller (which you can mitigate but buying small amounts at a time) it seems too good to be true.
What have I missed?
I wonder if there are any similar schemes for Virgin?