I feel that there are some basic questions that are useful to ask anytime you are buying something used.
1) how long have you owned it?
2) why are you selling?
3) is everything original?
4) if no to #3, what was changed (was it an intentional modification or a repair), why was it changed, who did the work, and what parts were used.
More specific:
1) Listen to all the drivers and make sure they work
2) if you have any concerns, ask the seller to open the cabinet and check that the drivers are correct. The second post in this thread has a list of the stock drivers for all the heritage speakers:
https://community.klipsch.com/index.php?/topic/212916-klipsch-heritage-series-historical-reference-data-v4-–-27-january-2016-by-groomlakearea51-with-additional-information-from-the-historian/
There are a few, well known sources for Klipsch OEM and aftermarket parts including JEM Performance Audio (the only Klipsch approved source for new capacitors), Crites Speakers (aftermarket), and ALK (aftermarket). If someone says they have parts from any of these sources, I wouldn’t be concerned about it.
Like buying anything used, you kind of have to size up the seller. If they seem legit and the speakers check out, it really comes down to price.
Price varies widely based on condition, what model you are buying, what version you are buying, and where you are buying. Heresys will be a fraction of the cost of Klipschorns. Original Heresy’s will be significantly cheaper than Heresy III’s. However, there shouldn’t be much price difference within the same version of speakers. For example, a Cornwall made in 1960 and one made in 1980 are both version 1. They have slightly different drivers, because Klipsch made some minor changes over time, but if the condition is similar price should be similar.
Hope this is some help.