I have noticed that most of the listings for Tesco Vouchers are stating that the seller will convert them and forward them to the buyer.
THIS IS NOT ESSENTIAL.
ANYBODY can enter the tesco voucher number when applying for the Clubcard Deal. This will then allow the Deal tokens to be sent directly to the applicant and in their name (in some circumstances).
You should also be aware that the Tesco Vouchers have an expiry date on them. Make sure that you ask the seller what this date is.
My friends and I have been combining our vouchers together for a long time now and when we have a substantial amount I make the application for the Deal tokens.