$600 seems way too high for this item. Series 1's don't usually even go for this much. I think the last series 1 pack I sold went for $500. Maybe rare stuff and higher end stuff has more of a market these days, I guess we will see moving forward.
How long ago did you sell your 1st series pack? I have only ever come across 2 for sale in the last 7 or 8 years. I couldn't guarantee the authenticity of 1, the other I purchased.
I think this is a combination of low demand and low supply. If the right person comes along and wants the item, the price has the potential to spike because they don't know when or if the next one will come along, however, at auction, a very rare & valuable item may not bring much if there aren't 2 qualified & motivated buyers that have seen the item.
I see this as an issue with older opc items, ludlows, 1st series tanbacks & 14th trifolds. If one of the few collectors sees a puzzle piece or sticker needed to fill a tough set, the price can go very high, but if the few collectors have the puzzle piece or sticker already, the sticker either becomes less important trade bait or is totally passed by. It will take a newbie opc/ludlow/trifold etc... collector for the price to spike, but when/if that collector comes along, it's a crapshoot.