Ya, I get what your saying. All my tanks that I have bought used were leaking when I bought them and I resealed them. Also when I reseal them I completely take the tank apart and meticulously clean the joints. Otherwise I buy new and even then I inspect every joint to make sure it looks good. I have seen some at the $1 per gallon sales that did not look safe.
I recently saw an article about a sw tank exploding at a university, and it was supposedly made by professionals. Yet the whole front pane of glass pealed off.
On another note, another idea that crossed my mind was a 6ft tall vivarium. Basically the 125 stood up on one end and a pane of glass siliconed, in what would be the bottom, to form the submerged portion and a removable acrylic upper portion for access. I have never seen anything that tall and want to build a vivarium.
That would make a freakin' AWESOME vivarium!
If I were in the market for a 125G tank only deal, and had no idea of your skills and abilities:
If I knew the tank was a leaker and was just re-sealed and NOT torn down and rebuilt, I would pass.
If I knew the tank was a leaker and was torn down and re-sealed I'd go $100 after a witnessing a 7-day leak test. I read this as a tank-only deal?
I fully understand everyone's thoughts on bigger tanks costing more and why, but there also needs to be considered the relatively limited market for 6 footers. Especially just a bare tank. Not to say there isn't demand; but people get squeemish about used tanks of those dimensions if they don't see it filled and running. In most cases they tend to be a big risk. And I fully acknowledge that new tanks of that size come with some risk as well. I bought a used 180G 7 years ago and specifically asked to see the tank running before it was torn down. Two trips, but worth it. Paid $200 for it, tank only.
It sure seems like you understand the process and do enough of this stuff. I suggest investing in a Toyo heavy glass cutter and make custom nanos as you were thinking. The Toyo cutter is one of truly coolest tolls I've ever owned.
But I still think the 6 foot tall vivarium would be just incredible.