It if's barred, DON'T send it to Mazuma - they will sieze it on behalf of CheckMEND, and unless you can prove to CheckMEND it belongs to you, they are not legally allowed to return the phone to you, and will not pay you a penny.
Not quite correct, but almost.
Envirophone and the like seize the phones, Mazuma checks the IMEI BEFORE you send it (not once they have it), so when you enter all the details the very last thing they do is to check the number, if it goes through, you get paid, and if Mazuma pings it back as clear you've also made "reasonable" effort to ensure the legitimacy of the goods.
If they tell you not to send it, DON'T.
Checkmend works well, takes a few minutes and a one-off check costs you less than £2, then if it IS barred you'll also know if it's because a network has blocked it (unpaid contract) or if it has been reported stolen.