Sorry you have to deal with this mess...
Perhaps you should try to work with seller to trace it with Hyatt rather than filing in SCC - given so many accounts of GC funds vanishing it is very possible that the seller sold it to you as advertised (and if not, i have no clue how you can prove it at this point...)
If that person still sells hyatt GC or anything else on ebay they should be interested in sorting this out rather than risking reputation... Perhaps hyatt will send a replacement if this can be traced as with many other reported cases.
It is really Hyatt we should blame for continuing problems with their currency - they should find a way to fix this...