CeX / WeBuy.com review: tablet purchase

Hi, I've just been in store to sell my Huawei mediapad M5 with bluetooth keyboard. I've been offered only £46 cash, yet £75 voucher, and you are selling it for £190! How can you sell it for £190 yet only give me £46 for it?
That's robbery and I'd be interested to see what the relevant authorities think of it. Even the voucher price is far above the cash price. The staff at CEX said its wrong and even they have never seen such a difference between value and offer.
Please can you correctly value this item for me? It's an A grade Huawei Mediapad M5, 128GB. Wifi only. It's only a month old.