Jett,
I use to work at an Ebay drop off shop that my friend owned (a couple of us were partners) and so I can give some info on what we did there.
Yes, many things came through the door that we knew nothing about (pipe organs, depression glass, car parts) and sometimes people could tell us about their items and others could not give much info. We used a lot of resources on the web to help identify things and when/why they were made/used. They are also a lot of good research tools to search closed listings to find out if the items would hopefully bring more than $50 (that was our criteria, to accept items that we thought would sell for at least that much and usually more)
We did use a fixed percentage on a sliding scale (the higher the price went, the lower the percentage). The commission was separate from fees. We paid the listing fees and then deducted final value and paypal fees from the clients profits as well. Fixed percentage seemed to work well and we had a nifty calculator that helped us to figure out what everyone gets if say their items sells for $100 (something like could be probably set up in excel)
Obviously our shop had its own paypal and bank account and boy did it make things easier! Believe me.
It is up to you to get a po box or not. We used the owners address until we got the storefront. I think she preferred to have money orders go there and in case any packages needed to be returned. That is up to you.
My opinion about the shipping thing is that I think you should do it. If your friends do not ship on time, not ship the item safely or well packed, then it is your feedback that is at risk. I liked having control over the process since it was our feedback at stake, not the clients.
Hope that helps, if I can help to answer anything else give a holler or you can reach me at my store.
Marcia