Check to see if the shelves can be laid flat. Unless you're displaying boxes like a cigar shop you'll be much happier with them flat. Flat = more room for boxes.
My tower has half the shelves angled and not movable, it's kind of a waste of space.
As for pricing, good question. I got my tower off craigslist a few years ago for $200. It did not include humidification though. I would offer about $500 if you really like the design, he may give you a price after that. It does look like a nice cabinet.