You need to allow for people to ship you silver to get shares. A good way for people with silver to get bitcoins.
Just a thought, change the terms a bit so you can offer generic .999, which these buffalos are. Maybe include a specific premium for eagles, and a discount for junk silver. For delivery, buyers get what you send and must pay the agreeded premium or take the discount.
Beyond quality issues it seems like the sender would also have to include the outgoing postage, insurance, and handling fee in order to keep the "1oz delivery included" definition. Also this could get to be a lot of managing work. It might be good to have 'registered' silver shippers working with the issuer/organizer.