Maybe the "bigger"* sellers could leave their full name/address/phone number with the staff? So, if something goes awry, they could be contacted directly?
*e.g.: single sale with $200 and, of course everybody who makes a "run"/permanent offer"
A wise man once said "Don't sell something you don't have"; I suggest to make a rule that people only pay when the item is ready to ship, And no "money in front" to pay a third party (like a machinist) in advance.