Did you consider letting the buyers and sellers do the leg work?
Setup a distribution center. Sellers who want to offload stuff scan the items and their details to you and you give them instant, fair quotes. If they like your offer, they bring the items to your center, and you check to make sure everything is as it was stated.
You put the items, their details and their new prices (with your margin markup) online for buyers to peruse. If interested, they'll come by and pick up whatever they want.
Like an online pawn store w/ local centers, except there's no pawning, just selling & buying.