Selling for others is very time consuming, and can be very stressful if isn't done right.
When I first started selling for others, I charged a flat rate fee of %35 per listing. I took out my fee and deducted the eBay fees from that.
I noticed that I wasn't selling many of my own items, and I was only making $2-3 per listing. I knew than that things had to change.
So now I also charge a flat rate fee of 50% per listing, all items are stored at our store and we shipped all the items.
Once people hear how much we charge to sell for them, they also prefer to do it on their own. We help them get started on eBay and it makes our life much easier.
Because we have a storefront, we try to stick with our local merchants. We make more money from them, then let say our friends. Selling for "friends" can be sticky sometimes.
We do have a different addy for money orders, cashier checks and such. This keeps our items apart from others.
Hope this info is helpful
and remember