I just traded in £60 worth of Tesco Clubcard Vouchers for Nationwide vouchers (£240 worth) and wandered if people were selling these vouchers on eBay.
To my surprise they go for between 2x - 3x the value of the vouchers!! eBay item 180385470741 for example, one guy paid £510 for £187 worth of vouchers! I would have thought that if you were paying nearly 3x the value for these vouchers, then any discount would be lost!
The thing I found with Tesco Clubcard Vouchers is that although they may give you up to 4x the value, you can only use them towards an inflated price. For example, I have found it cheaper to purchase RAC + magazine subscriptions using discount codes than using Tesco Clubcards.