fshy,
I too have been looking for a used set-up myself and have come to the following conclusions:
1. If the tank has been broken down, it's typically sitting in the garage/basement and will be available for about 25 cents on the dollar (someone just wants it out of there). If it's up and running, 50 cents or less seems to be a typical asking price.
2. The equipment is generally not recent vintage, but functional.
3. Stands tend to be trashed, but usually can be refinished quite nicely, especially if solid oak/pine.
4. Sellers tend to balk at proving there are no leaks since this would entail a few hours and plenty of water. Sadly, I would not take anyone's word that a tank does not leak.
5. Reef ready tanks seem to be a rare find.
6. Ask for dimensions before even taking a look. Many seem to be designs that would be difficult to maintain or won't fit in the area you have available. I've seen two 90 gallon tanks that were over 36 inches in height. My arms aren't quite that long.
7. It's a good idea to plan/budget based on the assumption that you may have to replace some of the equipment soon (heater, skimmer, pumps, etc.). Maybe you'll get lucky, maybe not.
All of the above said, you can find some GREAT deals from people "forced" to quit the hobby due to relocation, family addition, expense, etc. I'll defer to others and articles regarding the difficulty in moving a running set-up, but plan on a full day (more including prep time), bring a few friends, and reward them with plenty of food and beer afterwards. Good luck.