Are they safe to sell your phone

Yes.

I've used them a few times without any problems.

Same. Works well.

How long did it take to get your money

A week or so perhaps? Not next day, but give them a few days to receive phone, check it over etc. I don't recall it being a particularly slow service...

Same day they received the phone which has always been the day after I sent it.

I've sent a few phones to mazuma and get paid the same day they receive the phone. Must have sent about 5 phones over the years and never had a issue with mazuma

Once yes, once no

They scratched my screen and lost he battery cover once and sent the phone back to me faulty. I e-mailed them back and complained and they did give me my money after that - but a bit odd.

why did they send your phone back?

Because they said it was faulty and not in the condition that it should have been