Hi Ed, former moving professional here. I love the idea, but I think you have a lot of logistical obstacles to overcome. I thought I'd throw some questions at you to think about (and maybe you have already).
1. What will you do if something breaks? (As the saying goes, if a mover tells you he's never broken anything, he's either lying to you or it's his first day).
2. What will you do to protect yourself against mistaken/fraudulent damage claims? (i.e. Buyer/Seller claims you scratched the DR table, but you didn't).
3. How will you load your truck to maximize space efficiency? The more things you can store on the truck, the more things you can put up for auction. As a mover, the first thing I put into the truck is the last thing I take off [0]. But you can't do this as to the dynamic nature of your business. You always need access to everything on the truck...which is surprisingly harder than it seems.
4. What will you do with breakables (i.e. china, glasses, lamps etc)? Different animal than furniture.
5. If I'm selling a large ticket item, why would I use your service? For example, let's say I bought a couch for $2000 last year and now I want to sell it. I should be able to fetch $1000 for it. If I'm patient, I could probably get as much as $1700. You're taking half. $500-$850 is extremely expensive to list an item on the internet and move it across town.
That said, I wish you luck this weekend.
Edit: General Clean-Up
[0]: http://www.angieslist.com/files/packed-moving-truck.jpg