maybe there is none. when I took kiswahili classes, the teacher taught me "mwuzaji" saying it was the tanzanian/zanzibari writing. According to her, it was what she called the standard kiswahili (kiswahili sanifu). I think "muuzaji" is just a slight difference in spelling.
There are a number of nouns, usually in the M-/WA- class, where you'll find the singular sometimes starting with MU- and sometimes with MW-. I believe that the MU- form is old spelling and that it is the MW- spelling that is preferred.
I think that it is some wrong form. We have : 1) muuza ( wauza) 2) mwuzaji (wauzaji ) and they mean - seller(s) etc. The form - muuzaji - looks like a wrong mixture between these two forms.