My problem is that I buy everything cheap. When I then here that a collected edition is worth money, I can't help myself and I sell it. When the first Absolute Planetary was sold out, I heard you could make a lot of money off a copy. I had bought mine for $50 and sold mine for $270. That is a very good return. I have had the Ennis Punisher omni, the New Teen Titans omni and other stuff like that and sold them.
I think the Ennis book is reprinted one day. It, along with his MAX material, are the absolute best work of the Punisher and will always be in demand.
The Titans omni...you know, it had just about the worst binding of any book I have ever owned. DC has gotten better with their books and I wish they would reprint these but they did just release a TPB of the first bunch of issues and I would probably just rather have those than a badly produced omnibus.