My first metal detector was the GTA 500 which I replaced with a GTAx 500 after the GTA 500 failed. This was the only metal detector that I have owned that quit working. The GTAx 500 is easy to operate with notch discrimination, pinpoint, audio adjust and other common features that were helpful.
As my first metal detectors, the GTAx series worked well enough for me to be content for several years. I found a ton of coins, some silver jewelry and a few wheat pennies. Everything that I found with the GTAx was less than 6 inches deep so it is not a deep seeking metal detector.
Eventually, I started using a Garrett Scorpion Gold Stinger for coin hunting as it offered greater depth and better target separation than the GTAx. Having had the opportunity to use a variety of different metal detectors, the GTAx is not one that I would ever consider buying used.
Some of the disadvantages of using a GTAx 500 include the heavy bulky design for what you get. Now I do swing some heavy detectors, but that is because they are very powerful. I would not want to use a mediocre metal detector that is heavy when there are so many comparable alternatives out there.
Another weakness of the GTAx series is that I ended up replacing the two plastic battery holders several times as they were not well made. I also did not like the way the battery pod connected to the main housing. This connection was for all intents and purposes an old school telephone cord connector that put a lot of strain on the wires and was awkward to connect as the connector would slip inside of the upper rod and was at times hard to grab with one finger.
I still see these metal detectors pop up for sale occasionally on classifieds, but performance wise they are not better than the Garrett Ace series. In fact, the Ace 250 uses less batteries, weighs less and has some other features that the GTAx lacks. So before you pick up a used Garrett GTA 500, GTAx 500 or GTAx 550 metal detector used, instead opt for a Garrett Ace 250 or 350.