How are the sets numbered? Why not order the precious metal medallions un-numbered and number them when they arrive or when somebody asks for them? If you want to make them appear to be mint numbered, you can get the same type device as the mint uses. The methods usually used are either by striking selected numbers from a set of number punches or by engraving the medal surface with an engraving machine and a pantograph.

There should be no problem is disposing of unsold medals or even medals spoiled in the numbering process. Buy them close to melt value and make sure they are marked for the metal used and there are lots of refineries that will buy them close to melt value (maybe even the mint that makes them for the club?) You could even consider numbering the medals at the Convention as an extra attraction to purchase them. Maybe even "do it yourself" numbering by the buyer at the show?