Details to include on the cover letter for sourcing part-time tutoring/marking positions
Key things to include are 


Relevant experience - such as tutoring, teaching, marking,
Educational achievement - you have a PhD, presumably this is in a field related to the field in which you will be marking/tutoring,
Other experience relevant to the role, e.g. working with students in any role such as mentoring, personal development, group work, and
Some evidence-based statement relevant to the proposed work.


For number four, this is an opportunity to show that, for instance, you are able to work alone or as part of a team, that you are able to work to deadlines (important for returning marked scripts), and that you have excellent written and oral communication skills (important for feedback). Your comments in this statement must be backed up with evidence drawn from your past experience, work or training.