CEX should ban any resale of their vouchers but they wont as I guess the vouchers are a way to maintain or inflate the prices of the products they sell. It maintains the lower cash value they have.
I'm surprised the forum allows the sales of them given they're a competitor and the strict rules of not being able to sell for a profit as the purchase of these vouchers allow people to make a profit reselling elsewhere or making a saving from purchase from cex.
CEX is literally a fencing operation. I very much doubt their moral compass points anywhere near "ban resale of vouchers".