Modaco does not allow selling via the site, it's one of the 'reminders'.
I would caution AGAINST CeX. Under no circumstances post your phone to them. You could try a high street store, but no matter how good a condition your device is in you will never get the A grade price for it. This is only possible if the model is brand new. To be honest, all the online auction sites have their problems beside scam artists. Fees and commission can eat into the sale price - I was 'stung' by ebay/PayPal when I sold an iPhone a few years' ago and it left a sour taste.
If you want 'top dollar' you will have to sell privately. My ex-employer runs a 'for sale' board and maybe yours does too.