Look carefully at the voucher T&C - if the seller is obscuring them then don't buy.
The ones that are not worth buying are the vouchers for say '�100 off any ABTA tour operator's holiday'. You'll only get �100 off for a high minimum spend, and you'll probably find that you can get the same amount off at a local travel agent (rather than the one issuing the voucher) without the hassle of a voucher.
A seller will maybe ask for �5 for one of these but will be lucky if he sells it at all.