If I were to sell mine I'd probably be in the camp of marking it up. The reason being, I really like it and if I'm going to let it go then it needs to fund something else I want more. I guess if a person just didn't care for it and wanted to offload it I could see them selling it close to retail.
The argument for selling something that is no longer available for what you paid for it when it was available doesn't really hold. Supply and demand and all that. Take a look at any desirable, no longer made hand tools and the prices are several fold higher than retail.
I suspect as the years go on and these become more and more hard to find in good shape, pricing will go even higher. The comparable systems on the market today are very expensive and the vac sys when it was available was the best deal in town.
What I don't care for are the people who obviously bought it to turn a profit. Especially evident when the listing says it's their 'spare' or second unit.
Matt