I understand they appear for as little as $12k in the USA.
This compares with about £20k here, which includes 4k for installation.
Does anyone have any information on any internal changes that might be relevant, to make sure one is getting a unit which is internally current?
For example it is common in avionics to find, many years later, that anything before S/N XXXX is basically worthless as a service exchange because there was a completely new PCB installed after that. I recall this on the KX155A, KI256, KT76C.
Or a unit before a certain S/N cannot accept firmware updates past some version. Common on EHSIs.
Why would somebody no longer need a GTN750?
Most probably causes:
- demo unit from a dealer, ran many hours, touched by thousands
- aircraft had a fatal accident
- unit was returned as defect but the defect could be isolated so it is moved onto the next
(If you do that the please sell me your GNS430*W*s :-)
There are always used units made available to dealers at reduced prices and they are not prohibited from selling them as they wish for the price they wish and are not required to install them. These units are units that were returned to Garmin and can’t be sold for new. The customer may have insisted on a replacement unit. They go thru the full manufacturing process and are as good as new. They usually have a reduced warranty period of 1 year. I bought my GNS530 that way back in 2001 and have only returned it to Garmin for service for the upgrade to WAAS. It has been trouble free for 13 years. You can get some great bargains from this program, they are sold with a warranty and are used, but as good as new in every respect.
Edit: I sold my GNS430 to install the GNS530 and got $2000 more for it on Ebay than my 530 cost me.
Why would somebody no longer need a GTN750?
Most probably causes:
- demo unit from a dealer, ran many hours, touched by thousands
- aircraft had a fatal accident
- unit was returned as defect but the defect could be isolated so it is moved onto the next
They can’t be sold as new or just put back into regular customer shipments if they go thru Garmin. Even if it was a demo unit, these are fully refurbished if they are returned to Garmin, even if not, a Demo unit often gets less use than a regular used unit and remains in great condition. Fingerprints do wash off. A fatal accident will likely ruin the avionics, but an insurance total can result if the aircraft is involved in a gear up or ground incident, particularly if the aircraft is under insured and has great salvage value.
I would want to know for sure where it had been. I would not want e.g. a unit which came out of a crashed plane, because the factory tests will not discover an intermittent connection. Factory tests – that do not include functional testing under vibration – assume that the product is basically sound mechanically and e.g. doesn’t have a crack running through the PCB, broken connectors, etc.
At work (industrial electronics) we throw away nearly everything which comes back – because we have found lots of customers lie about what they were doing with it, and you can have subtle damage. Not just mechanical but electrical. We have even had customers send back products which were assembled from several others! We found out because the S/N on the label didn’t match the one in the EEPROM 
If you send that back out and there is a problem you will have two pissed off customers instead of one. Luckily very little ever comes back so the economic impact is nil, but I am sure Garmin will not throw away a GTN750 (worth a few hundred $ to make) which they know was in a bad crash but which passes all the factory tests and doesn’t have visually obvious damage.
