That's really a decision that only you can make.

How much is your time worth to you?

How much money to you require to make it worthwhile for you to be doing eBay rather than something else?

ie: put simply it's Opportunity Cost. What do you get for what you have to give up? Is what you get worth what you have to give up?

If working for someone else, you really do need to nut out a business plan. There are so many things to consider. Including how much you like these people and if you are just helping out a mate or trying to generate some cash

Then consider if you want to be paid per item whether it sells or not or a percentage of total sales or even a combination of both.

From a business angle: maybe charge $11 per initial listing as you should be able to work up 5 or 6 listings an hour. then maybe $2.20 for each relist?
 plus a percentage of sales or a minimum set price up to say $550 and then a percentage thereafter?

Remember that your services will be subject to GST

dunno really, it's a personal thing and really depends on your motivation and what you perceive your time is worth.