You don't have to use eBay postage, but any postage service you use will charge you.

So postage costs are irrelevant.

You are paying store fees ($27 or $60 per month) plus less than 10% on turnover, which includes postage.

If that's $1000, then you are turning over $10k per month. Only you know. As Lyndal said, look at your invoice.

Try renting a B&M shop - then you will know what overheads are.